Being in the room where the decisions are made is the only place for any ambitious politician to be. Invited to the meetings, consulted on the key issues of the day, never a few feet from the party leader’s side – now that’s power.

But there is one politician in the room who is unpaid, performs a role that is frequently mocked by the press, and, in return for the privilege of being there, is bound to vote loyally with the government. Meet the parliamentary private secretaries (PPSs), or, to give them their unofficial shorthand title, Westminster’s bag-carriers.

Still, if you’re going to carry anyone’s bags, they might as well belong to the most senior person in your party. Gavin Williamson, PPS to the prime minister, goes as far as to describe it as “the best job in the world”, though presumably David Cameron disagrees. Williamson, Conservative MP for South Staffordshire, has already chalked up stints on his CV as a PPS to former Northern Ireland minister Hugo Swire and transport secretary Patrick McLoughlin before No 10 came calling last autumn.

“It was a total shock”, says the affable and remarkably youthful-looking 37-year-old of his invitation to Downing Street. “Actually, I saw Gavin Barwell there, so I assumed there had been a mistake on the switchboard and they had called two Gavins. He’s an incredibly capable individual, so I supposed I had been the mistake…”

He wasn’t: Williamson was appointed as Cameron’s new bag-carrier.

Williamson occasionally bumps into Simon Wright, the 34 year-old appointed as PPS to Nick Clegg last Christmas Eve when Duncan Hames, his predecessor, decided to create more time for his baby son.

“I had no idea that Duncan was going to be stepping down, and I wasn’t expecting to be top of the list as his successor,” says Wright, a similarly upbeat character – perhaps it’s a prerequisite for the job? Wright had served as PPS to shadow education secretary Sarah Teather and education minister David Laws, and got the call while walking with his family in Malvern on Christmas Eve, “which gave me one more thing to celebrate over Christmas.”

Ed Miliband has two PPSs, both of an older vintage than Wright and Williamson. One, Karen Buck, has been an MP since 1997 and occasionally served as a minister during the New Labour years. The other is Wayne David, a former MEP, an MP since 2001 and another with ministerial experience.

You’ll see all four of them around the parliamentary estate, a few paces behind the boss, talking with MPs, or sitting behind their respective party leader at PMQs. But what exactly do they all do?

“There’s no hard and fast job description to the role – it’s basically working to make sure he [Miliband] is as succesful as he possibly can be,” says Wayne David, adding that a PPS provide a “crucial link” between leader and party, as well as playing a “big role in term of preparation for PMQs.”

Williamson describes the job as about being “a very candid friend... You’ve got to go out there and understand what people’s concerns are in terms of the parliamentary party and try to bring that back to the prime minister. Equally, you’ve got to try and articulate what the PM is doing and thinking, and where he is at. It’s sometimes to deliver bad news, hopefully sometimes to deliver good news – it’s also trying to touch on the general feeling as to where things are.”

With 56 Liberal Democrat MPs compared to 303 Tories, Wright’s job of keeping his “ear to the ground on absolutely everything” is a little more straightforward. “By and large, it’s probably more easily done in my party than it is in the two bigger ones…” he admits.

Buck, who was appointed last April, sets out to be “a conduit between the leader’s office and the parliamentary office, keeping Ed informed with what people are saying and thinking, and being a channel of communication with the parliamentary party, to pass on what MPs want to say to him.”

If it all sounds easy, it isn’t. When Margaret Thatcher lost the Tory leadership in 1990, her PPS, Peter Morrison, was blamed for failing to do enough to canvass backbench opinion. Being candid, it seems, requires care.

“I remember the prime minister saying, ‘You mustn’t be at all nervous about telling me bad news’,” Williamson recalls. Good advice, of course, but the PM hadn’t factored in his PPS’ relentlessly cheery demeanour. “After I did tell him a bit of bad news, he pulled me up and said, ‘You shouldn’t look as if you’re enjoying it!’”

Wright agrees that it is “important” to tell the leader what he may not want to hear, because “there’s no value as a PPS if I’m not able to hear the honestly-felt opinions of my colleagues.”

At the same time, a PPS is a head prefect – part of the class, but with a foot in the headmaster’s camp too. Surely the tearoom falls silent whenever the PPS falls in?

“I never repeat all the banter and the jokes that I hear, but you’re there to be the eyes and ears,” Williamson replies. “Most people realise that you’ve got a job to do and hopefully find it useful.” David agrees, saying: “People are very honest, They want to be helpful. They make all kinds of suggestions.”

Wright insists his presence in a room is hardly likely to stop his colleagues saying what they think. “We can speak frankly to one another. I don’t get the sense that people stop talking or hold back. Lib Dem MPs, as a breed, tend not to hold back when they’ve got something to say.”

Buck, who doesn’t like to use the ‘eyes and ears’ description as “that’s the whips’ job”, laughs at the suggestion. “I haven’t noticed… people want to say whatever they want to say. You’re not just there to listen to good things; that would be pointless.”

The working day is a busy one for all three, with regular meetings with the party leaders. Williamson arrives at his parliamentary office at “some ungodly hour in the morning”, before racing across to the Commons tearoom. That’s all before the daily 8.30am meeting in Downing Street. He says he is thrilled to have “the opportunity to sit and witness what goes on and how the heart of government really does function – it’s an amazing training ground”.

The frantic pace of the job – Williamson is back and forth between No 10 no less than three times a day – has other benefits. It doesn’t look like he needs to shed any pounds, but he relishes the work out: “I do absolutely no exercise, but I seem to spend my whole time running between No 10 and the House of Commons. Some days you’re a like a yo-yo between the two places.”

Wright also clocks in at 8.30am for the first meeting of the day with Clegg at the Cabinet Office. “It’s his core team, his advisers and his personal office, but l’m the only one there who’s a parliamentarian,” Wright explains. “They don’t have the access that I do to the views of my colleagues.”

With more than a foot in the door, why don’t the leaders’ PPSs make themselves part of the furniture in their leaders’ offices? Williamson has a hot desk available, as does Wright. It must be tempting to stick around, but Williamson shakes his head. “You’ll end up being captive,” he counters. “If I’ve got to sit down somewhere I’d rather sit somewhere where a fellow MP might be able to find me, as opposed to behind the iron gates of No 10. My job isn’t to sit in No 10; my aim is to listen to people, and they’re not all sat round at No 10.”

Wright agrees. “If I need to sit down, I’ll find somewhere to perch, but by and large the most added value I can bring to the operation is being in parliament and hearing colleagues’ concerns – I can only do that here, not in Nick’s office.”

Buck and David, however, do have desks in Miliband’s parliamentary quarters, which David describes as a “useful anchor inside the leader’s office”. Not that they are always to be found there. “We drift in and out,” says Buck. “I don’t regard myself as physically based there, but it’s a very welcoming office. Some days you can be there quite a lot – sometimes your feet hardly touch the carpet.”

Back in Downing Street, Cameron’s close-knit team is dominated by public school and Oxbridge career politicians. Does Williamson, an alumnus of Scarborough Sixth Form College and Bradford University, bring a balance to the group? After all, he is probably the first prime ministerial PPS to have run a pottery business before entering politics. “Any time the prime minister wants advice on pottery, I’m the only one in No 10 who’ll be able to give it,” comes the light-hearted response.

But doesn’t he have the advantage of having knowledge of a world with which few in Downing Street are familiar? “I’d like to think it is [an advantage], but there’s a vast variety of people within No 10 from a whole range of backgrounds,” comes the diplomatic reply.

Wright, a former teacher, brings something different to the table. “The need to narrow the attainment gap, the pupil premium, was something Clegg personally championed before he became party leader, and I feel that, because of my professional background, that’s something I can bring to his operation.”

Buck’s appointment was interpreted in some quarters as the deployment of a left-leaning MP to reach out to that part of the party, but she rejects the suggestion: “People come from a range of different political perspectives and priorities, but there’s no fundamental divide of ‘there’s the left and there’s the right’ – it’s not like that. You have to know as many people as you possibly can.”

However, pointing to her political experience, Buck notes that “it’s very important that you have a collective memory as well as new insights and new energy – it’s important that you keep an awareness of both sides. Clearly, Wayne and I have a number of years of different levels of experiences within the party, and that’s a good thing.”

Being a PPS also brings added political pressure. Say anything out of line with your party, and a rebellion – at least in the eyes of the political press pack – has been triggered. Buck says she feels no added pressure, but she has been stung, as comments she made last November on welfare reform were spun into an attack by a “top Miliband aide.” She rolls her eyes. “The media occasionally likes to make a little bit of mischief, but I don’t think being a PPS makes me a higher risk than anyone else on the frontbench.” Of course, says David, “you have to be careful, but it’s not a problem – my views are instinctively the views of Ed Miliband.”

Williamson is also well aware that “you would always want to be a bit careful about how you weigh into an argument,” while Wright notes that it is “common sense” for a PPS to know what to say and where to say it, even if “it would be more surprising if PPSs felt they could never express their personal opinions.”

The need to proceed with care must sometimes feel like something of a rough deal; the rest of the time the humble PPS isn’t exactly treated with respect by the media.

“If a PPS resigns, the press would make it a much bigger role. The rest of the time they’re very derogatory about them.” Williamson shrugs, but isn’t it a little galling to be labelled as a mere bag-carrier?

“There is no more noble an art than carrying a bag. I could almost muster two…,” he replies with a smile.

Williamson has even had to endure a put-down in public during prime minister’s questions, when John Bercow, the Commons Speaker, told him his “role is to nod his head in the appropriate places, and to fetch and carry notes – no noise required.” Surely that hurt? If it did, you would never guess, as Williamson breaks into an even wider smile. “I wear it as a badge of honour,” he declares of his run-in with Bercow.

Wright accept that parts of the job may not be “high level activity”, but that doesn’t make them any less important. “At DPM questions I have to do a lot of the work in terms of lining up colleagues to ask questions, to make sure that Nick is aware of any issues that might be coming, and to generally offer encouragement. It’s important work.”

Being at a slightly different stage in her career, Buck is quite happy to be called a bag-carrier. “It doesn’t bother me in the slightest,” she insists. “I loved being a backbench MP doing select committee work, I’ve done frontbench roles, and it was my choice to do this. If I thought it was meaningless or demeaning, I wouldn’t do it. I enjoy it. I don’t care what people describe it as.” David is the same, defending a job which “is not a menial task”.

Williamson jokes that he is a “multiple PPS – I’ve obviously found my level, and I’m very comfortable at that level.” But the position of PPS is often seen as the first step towards higher office, so are Williamson, Wright and Buck destined for greater things? A look back over the last 15 years or so would suggest not.

Of the seven PPSs to serve during the New Labour years, David Hanson, a PPS to Tony Blair in the first half of the last decade, spent time as a minister at the Home and Northern Ireland Offices, while Desmond Swayne and Sam Gyimah, Willamson’s two Tory predecessors, currently reside in the government Whips’ Office. But great offices of state have not been troubled.

Look a little further back, however, and it’s clear that being a PPS to a PM can be the first step towards the top. Anthony Barber, a future chancellor, served as PPS to Harold Macmillan, while Robert Carr was a PPS for Anthony Eden – supposedly his tennis skills caught the Tory prime minister’s eye – before rising through the ranks to become home secretary, and, in the week’s inter-regnum between Edward Heath’s resignation and Thatcher’s election, briefly a stand-in Conservative leader. A future prime minister has even done a stint: Alec Douglas-Home, head of the country in 1963, had served as PPS to Neville Chamberlain for the first 18 months of the Second World War. More recently, however, getting a foothold on the greasy pole hasn’t proved to be the leg-up it might be.

Wright and Williamson are well honed in the “focusing on the job” stock answer, while Buck has “never set out on a career path – if you thought about being in parliament in a prescriptive way, you’d probably be disappointed.” As for Wayne David, being a PPS, he says, “is a worthwhile job in itself and something I feel I can make a distinct input too.”

For now, Wright is merely hoping his job title will help in a more immediate battle: to hold on to his parliamentary seat.

With a wafer-thin majority of 310, he needs all the help he can get. “My constituents are aware that I have this role, and it can be a benefit for Norwich to know about this route through to the deputy prime minister,” he argues. “Seeing that they’ve got a local MP who is at least thought of highly enough to be offered this role is a positive thing. Clearly some of my colleagues think I am doing an ok job…”

Until then, there are concerns to be heard, frank discussions to have, and a lot of running around to do. It’s a peculiar and particular job. Perhaps they should share tips?

As coalition partners, Williamson and Wright are in occasional contact. “There’s a relationship there which is mutually helpful,” admits Wright of the shared information between the two, but Buck, as an opposition representative, does not join their conversation. Williamson smiles, and says he is “always happy to open up cross-party dialogue”, while Buck has a more sociable suggestion. “Maybe we should have a PPS’ dining club?” she proposes.

Now that really would be a room worth being in. Just leave the bags at the door.