It’s a strange and unsettling experience to wander past people working at the BBC’s New Broadcasting House headquarters when you’re following a television crew that’s making a comedy about people working at BBC’s New Broadcasting House headquarters. Even more so when one of the actors, Hugh Bonneville, is very famous, and yet everyone is pretending not to notice him.

Bonneville, star of Downton Abbey and the Paddington movies, plays BBC Head of Values Ian Fletcher in W1A, the show that grew out of London Olympics satire Twenty Twelve and is now on a third series of merciless digs at the BBC management culture. Fletcher, an essentially decent man lost in the labyrinthine language and customs of the Corporation, has become a talisman for our baffling times. “You see Ian Fletchers everywhere,” says Bonneville. “In all buildings, offices and institutions.”

Today Fletcher is at the centre of a panicked kerfuffle with Anna Rampton, Director of Better (Sarah Parish), and Simon Harwood, Director of Strategic Governance (Jason Watkins). It’s no spoiler to reveal that a crisis involving accountability, ethics and ego is under way at the BBC, but when he stops for a coffee with me Bonneville claims W1A is created with enormous affection (which, as it’s made by the BBC, shouldn’t surprise). “It’s a bit like your favourite auntie who drinks too much,” he says. “You just wish she wouldn’t. And there is a cri de coeur in the final episode from Fletcher about the value of the BBC and the importance of it, and do we actually want to end up with just Fox News?”

Bonneville is still, perhaps, best known for playing Robert Crawley, Earl of Grantham, in Downton Abbey between 2010 and 2015, a career-defining role he is immensely proud of. “I get dozens of letters about Downton Abbey, with people saying, ‘It got me through a bad time,’ or, ‘We watched it as a family and that was a rarity,’ ‘That episode made me laugh,’ or, ‘This episode made me think about this.’”

Hugh Bonneville as Robert Crawley, Earl of Grantham in ‘Downton Abbey’

If Julian Fellowes gets his way we’ll be seeing the Earl of Grantham again in an upcoming Downton movie, but despite being in great demand Bonneville reveals himself to be as insecure as any beginner. “You know, you’re always convinced that the next job will be the last one.” And yet they keep coming. Bonneville is about to appear in Paddington 2, with Hugh Grant, his Notting Hill co-star. “Hugh’s probably had Botox as he doesn’t seem to have changed at all,” Bonneville says. “But we were reflecting on the fact that it’s nearly 20 years since we did Notting Hill; it made me feel very old suddenly.”

Married to Lulu Williams since 1998 (the couple have a teenage son, Felix) and “sliding into the bath of middle age”, Bonneville claims to be more confused at 53 than he was at 18. “The older you get, the less you know,” he says. “But I have a total acceptance of life.”

Bonneville has been an actor for much of that life, apart from a spell in his late teens. “It was around 1982 and I had to save £400 to travel across Africa to India,” he says. “I tried working in a pub up the Edgware Road in London. It was bleak, really bleak. There was a three-legged dog in the corner and one bloke at the bar and the room was full of cigarette smoke. It was depressing beyond hell. I thought, ‘However much I want to fly across the world, I can’t do this.’ That’s why I took up the toilet cleaning, at a legal firm in Marylebone.”

Hang on – the most famous aristocrat in the world pulling on his rubber gloves and pushing a brush round the pan?

“Well, they provided the rubber gloves and brushes. I’m still pretty good at selecting various different loo brushes these days. I’m known as the Poo Wallah in my house. I’m quite bombproof when it comes to cleaning stuff.”

Bonneville planned to work as a barrister if his acting career didn’t take off. Would he have been any good? “No, terrible. My mind wouldn’t have focused. I wouldn’t have been able to retain case law and all that. I think I’m a shirker rather than a sherpa. I was attracted to the performance side, rather than being a sensible solicitor with a steady income. I used to spend time watching the law courts when I was a teenager. I would go along like a nerd in the holidays and just find out about it. In retrospect, I was probably watching the theatricality.

“But I was also enjoying the lack of theatricality. I’d watch Lord Denning giving his judgement on some really tedious tax case as Master of the Rolls. I was captivated by the slow pace of it, that a system can function with such glacial slowness and yet somehow work. And I found Lord Denning’s demeanour and accent attractive.”

Denning, regarded as the greatest British judge of the 20th century, was a draper’s son who went to a national school for the poor. Bonneville, conversely, went to public school at Sherborne in Dorset before reading theology at Cambridge and attending the prestigious, but now defunct, Webber Douglas School of Acting in London. “I feel incredibly privileged. I wasn’t born with a silver spoon in my mouth but I had a nice set of crockery.” He likes to help those “who didn’t have the start I had. My parents spent a small fortune on my education, so I try to give back to charities and initiatives.”

Bonneville is still in some awe of his parents. “My mum was a nurse before I was born and I’ve never known someone work as hard as my dad. He was a surgeon and didn’t come from a privileged background; he worked his socks off. He literally saved lives. That gives me a healthy appreciation for the absurdity of what I do. I can’t deny that it gives great pleasure to people, I know it has its value, but it’s not world-changing. It’s prancing around in tights, basically, pretending to be other people.”

The cast of W1A (L-R): Simon Harwood (Jason Watkins), Izzy Gould (Ophelia Lovibond), Tracey Pritchard (Monica Dolan), Will Humphries (Hugh Skinner), Ian Fletcher (Bonnerville), Anna Rampton (Sarah Parish), Siobhan Sharpe (Jessica Hynes), Neil Reid (David Westhead, Lucy Freeman (Nina Sosanya), David Wilkes (Rufus Jones), Jack Patterson (Jonathan Bailey) (BBC/Jack Barnes)

He calls W1A “a contemporary, affectionate comedy”. However, the show’s central device is an old one, which Bonneville describes as “people being promoted above their abilities. We all know people who shouldn’t be in the job. I think the collective jaw is still on the floor that a non-politician can take over the most powerful country in the world.”

When I point out that British actors often criticise Donald Trump, Bonneville verges on shirty. “I’m not a great political animal,” he says, “but the fact is, if you are in the creative arts you’re not allowed an opinion. A person who lives on my street might say something and not be noticed, but if you happen to be on the telly then you get stick for it. We are vacuous idiots with no thoughts of our own.”

It’s not the conciliatory way Ian Fletcher would respond to a remark he didn’t like. Bonneville would not be an enthusiastic Head of Values in real life; the idea of working in an actual office horrifies him: “I just wouldn’t function in that kind of environment.”

Although, acting isn’t free from the troubles that beset an institution like the BBC – there are monsters aplenty. Especially, says Bonneville, theatre directors in the 60s, 70s and 80s. “I worked with one who was famous for having a whipping boy. I wasn’t his whipping boy, but I observed it happening and I remember speaking out and receiving some ghastly old fogey-ish comment like, ‘Fighting back from the ranks, are we?’ I said, ‘No, I’m just asking you to give us all a break.’ And he calmed down. If you stand up to bullies, hopefully with a bit of wit and charm, then they back off. While I take my work seriously, I try to keep a light tone on it.”

But Bonneville says he would like to wind that work down a little. “I want to reflect a bit more and not necessarily grab the very next job that’s coming along, even though the instinct will always be there to do so. I’d like to be a bit more calm and selective and maybe generate some work of one’s own as well.”

What does Bonneville imagine he will do in the time he has left?

“My God, that sounds like a death sentence… Maybe writing.” A memoir perhaps – From Poo to Paddington, I suggest. “No,” he says, “I’ve got it: Lessons in the U-bend of Life.”

Interview by Michael Hodges

W1A returns on Monday 18th September at 10pm on BBC2