"Great TV shows are like rock 'n' roll bands - you mess with the line-up at your peril," says Dennis Waterman. And he should know. Having starred in four of the greatest buddy shows of the past 40 years, he's quitting New Tricks as the final season begins, for one clear reason – you mess with the line-up and the magic goes.

“This is no slight on the new cast, but everything did change really dramatically and suddenly when Amanda and Alun left,” he explains, describing his sense of loss after Redman and Armstrong followed James Bolam off the show. “I’d already signed up for the next season. I did ask Amanda if I’d look sad carrying on after everybody else had quit and she said, ‘Don’t be daft.’ And then my wife started buying another bleeding house and I had no option. But it wasn’t the same.”

He’s keen to stress again that he enjoyed working with Nicholas Lyndhurst, Tamzin Outhwaite and Denis Lawson – “When Denis joined it wasn’t too sharp a shock because Amanda and Alan were still in it, and he knew us and it carried on” – although elements of the new characters slightly bugged him.

“Nick’s character, Danny, is really quite weird,” he points out. “With Alun’s character, Brian, he was memory lane – he remembered stuff, all very simple. Nick’s playing a genius who knows everything, which I was surprised by. There was one episode where overnight he teaches himself to play football, and I thought that was ridiculous.”

The current New Tricks cast (l-r Nicholas Lyndhurst, Denis Lawson, Tamzin Outhwaite and Dennis Waterman) 

When Waterman announced his decision not to return for season 12, the producers asked him to film a final story, to allow them to properly write Gerry Standing out. "They pointed out that everybody so far has been written out – and that Julian Simpson was available to write and direct those episodes, so I said OK!" He gives a quiet laugh.

Simpson is the New Tricks writer who took to Twitter to express his fury when the old cast described to RT how they improvised lines on set and helped shape the script. It’s clear the spat is over; Waterman is full of respect for Simpson, and Gerry’s final story is an epic tale of honour, love, loyalty and family, with Life on Mars-style flashbacks and a finale that’s pure primetime spectacle. But the Clapham-born veteran can’t resist adding his codicil: “Julian went crackers and said he never got any help. But we’d run scenes, say ‘This doesn’t seem right, wouldn’t it be better this way?’ And Julian would say, ‘Yes, that’s a good idea,’ and the script would change.”

He’s saying all this, he points out, just to underline how special New Tricks was. “What astounded me was how total strangers would come up to me and say how obvious it was that we loved working together," he recalls happily. "You think the audience shouldn't be able to see that deeply into us. And they were right – we were all having a great time together, but I was always amazed that other people saw it so clearly."

New Tricks launched back in 2003 – the same year as BBC3, Little Britain, Peep Show and the first gay kiss on Coronation Street. Doctor Who’s reboot had just been announced, and the idea of a TV show about craggy-faced and foul-mouthed old detectives with bad hygiene and hopeless emotional problems called out of retirement to solve cases left open for decades was almost unimaginable. The show doesn’t even figure in Wikipedia’s account of British TV in 2003. Indeed, when ambitious superintendent Sandra Pullman (Redman) first took charge of the Unsolved Crime and Open Case Squad it was only for a one-off pilot.

But the reputations of Waterman, Bolam, Armstrong and Redman pulled in enough viewers for the BBC to grudgingly commission a full series – and then something strange happened. These curmudgeonly coppers, baffled by new technology, hating modern policing methods and clearly in no state to mount a rooftop chase, proved gripping to viewers across the globe.

In a welcome reversal of British TV’s HBO obsession, Americans love the show: “We get people from all over – even in Indiana...” Waterman still seems slightly incredulous. “I mean, they’re still playing banjos in Indiana...” He lets out a huge guffaw. It’s clear he’s far more relaxed than the last time we met, on a shoot for the Radio Times cover announcing Denis Lawson’s casting in 2012. Then, he was concerned about everything, checking every bit of the day was working properly. He was like that on set – always keen to get it right.