In some ways, National Treasure is a story you’ve heard almost every month since Jimmy Savile died – an old-school TV star is accused of historic sex offences and winds up in court.

The beloved entertainer in Jack Thorne’s drama is Paul Finchley, played by Robbie Coltrane, who faces a public trial as well as they anger and scrutiny of his wife Mari (Julie Walters) and daughter Dee (Andrea Riseborough).

Coltrane is keen to point out that, as his character repeats more than a few times, Finchley is not Jimmy Savile. “If you wanted to write about Savile, you’d want to be asking why the hell didn’t anyone do anything about him earlier?

There was a wonderful article in The Independent where they named the five people that should have come forward – BBC execs, people who should or could have known...

“Everyone knew Jimmy Savile was a creep. Everyone. I never met him but you’d watch him and you’d feel your skin crawl. What this is about is what happens to you when you’re accused – guilty or not guilty doesn’t even come into it until the final episode.”

One key question the drama lingers on is whether the cops are correct in leaking the names of suspects – with Cliff Richard and Jimmy Tarbuck both named then exonerated, the risks to the innocent are clear.

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Coltrane has no doubts: “Yes, it is a dichotomy – who do we protect the most? The potentially innocent, by keeping them anonymous, or the damaged victims who suffer alone and who wouldn’t come forward unless they saw others were? In my mind, it’s clear. It’s the victims who deserve the most protection.”

Coltrane’s career began, ironically, as an alternative-comedy young gun taking down the careers of the light-entertainment performers of the 70s and 80s currently at the heart of Operation Yewtree.

Readers with a long memory might recall him on A Kick up the Eighties – a curiously uncelebrated BBC1 sketch show that introduced Rik Mayall and Tracey Ullman to television.

One skit saw him playing a truly terrible Butlins nightclub comedian trying to end his set on woefully unfunny topical satire.

He’d leaf through the paper trying to riff on headlines and delivering bland non-punchlines – “Denis Healey, the man with the huge eyebrows, has gone to Budapest to see how they run the unions there. He won’t see much, not with those eyebrows... don’t throw that – it’s too heavy...”

“I used to write that – just before you say anything rude about it,” he says cheerfully. “He was one of those ropey old comedians who goes on about his wife and golf and had a shirtfront that looked like a Viennetta. The kind we booted off stage in 1982.

After that I got sent to interview Les Dawson to give the old guard a good kicking, and I completely fell in love with him,” he laughs. “I was trying to explain why we didn’t like the mother-in-law gags and he said, ‘You don’t understand because you’re not working-class and not my age.’

“Working-class men, when they got married, couldn’t afford to rent or buy a house so they would usually live with her parents. The mother-in-law ran the house, so you were living in somebody else’s house and were told what to do.

And, of course, Dawson’s gags were great – ‘I won’t say she’s ugly, but when she opened the door, the mice threw themselves on the trap.’”

In National Treasure Coltrane’s fictional double-act partner Karl is played by Tim McInnerny – who excels at playing the concerned, slightly more successful, scene-stealing member of the duo.

Coltrane and McInnerny both appeared in the comedy series Blackadder the Third back in the 1980s (although in different episodes) when Coltrane played Samuel Johnson.

“Rehearsals on that show were just staggering – we used to go there just to watch,” he recalls. “I’d pay to go and watch another rehearsal with that crew – just to hear [producer] John Lloyd making rude remarks to Stephen Fry, or Stephen and Hugh Laurie mucking about.

"Rowan Atkinson, of course, is a great man – but I used to tease him about him mispronouncing his own name. It’s Rowan like “cow”, because it’s a Scottish name. Now he’s doing Maigret – everyone plays detectives in the end.”

It was Coltrane’s own move to detective – playing Fitz in ITV’s Cracker (1993–2006) – that punted him to the A list and helped him cut down on the boozy Soho life that was the comedy world.

Just before Cracker he met sculptor Rhona Gemmell, had a son, Spencer, now 24, and daughter, Alice, now 18. For a while he moved into a remote farmhouse near Loch Lomond. In 1999, the couple married but four years later split.

All the time his career was moving at a rapid pace – Cracker led to Bond to Potter and secured his reputation for dramatic range. He’s done well since, although he’s keen to point out, “I’m certainly not in the ranks of the super-rich. Let’s get that out of the way. I do not have anything like that kind of money.”

He shifts uncomfortably in his dressing-room chair as the questions move away from his rolling anecdotes and into his family and finances.

A flash of Glasgow Robbie emerges when he decides it’s time to move on – “It’s probably the only thing I share with Paul Finchley,” he smiles briefly. “I’m not keen on putting my family in the media spotlight.”

Then he’s called to set and, as he walks through the courtroom towards the witness stand, everyone from crew to extras seems to know him and all want to say hello. He rests on his stick, looks strained and tired but gives a big grin and has a friendly word with everyone.

It takes him 15 minutes just to get across the floor. In his own sweet way, Robbie Coltrane is becoming something we’ve waited for since the horror of Yewtree began – a national treasure we can finally believe in.

National Treasure starts Tuesday 20 September at 9pm on Channel 4