The Crown's Alex Jennings admits he "won't like" watching his replacement in the Netflix series

"There's a really good scene – ten days before the Duke of Windsor died the Queen went to visit him, and I really wanted to do that. But they've got some old bloke coming in."

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He adds: "I was disappointed by that – but I totally get it."

The identity of the "old bloke" has yet to be confirmed. But as the whole cast ages up, and as Olivia Coleman takes over as Queen Elizabeth from Claire Foy and Tobias Menzies replaces Matt Smith as Prince Philip, Jennings will have to watch on as another actor plays "his" role.

Won't that be weird to watch on screen?

"Yeah, I won't like that at all," Jennings says. "Because it was such a fantastic thing to be a part of, and it was such a happy experience."

Many TV drama fans will also recognise Jennings from his role as scheming German uncle King Leopold in Victoria (yet another royal role), or as Peter Bessell MP in the BBC's brilliant A Very English Scandal.

He's also played a younger generation of royal in The Crown screenwriter Peter Morgan's 2006 movie The Queen, as Prince Charles – with Helen Mirren as Queen Elizabeth herself.

"It's weird, for them, how they, the ones that are alive, are out there on the television screens having their lives reenacted," he reflects. "Very strange. And they can't respond to it."

He adds: "I have met Prince Charles and he's never said anything. He sort of looked at me a bit oddly. He's the patron of an actors' charity, the Actors' Benevolent Fund, that I'm a trustee of, and it was shortly after The Queen came out.

"I was really nervous. He just looked with a sort of, 'I know what you've been doing...'"

On ITV, Jennings currently stars in series three of Unforgotten as Dr Tim Finch, a successful GP and a suspect in the murder of a teenage girl. But unlike many of the actor's recent characters, Dr Finch is neither royal nor overly posh.

"He's done alright. He's very very comfortable," Jennings tells us on a break between shooting his scenes on set in Buckinghamshire. "Happily married, second wife, two children, and I was very attracted to it as well because it meant not playing a member of the royal family. And wearing modern clothes – which I never do."

It's certainly a change of pace.

"I had to de-posh myself," the actor explains. And that was harder than you might think: "I'm not posh at all actually, but I've sort of acquired this poshness as the years have gone on and I have done a lot of posh."

Unforgotten, which stars Nicola Walker as DCI Cassie Stuart and Sanjeev Bhaskar as DI Sunny Khan, is now on its third series and its third murder case. Written by Chris Lang and directed by Andy Wilson, it is a far cry from other police dramas; for a start, there are no maverick cops or car chases.

"It's very naturalistic and that's a bit of a test for me," Jennings says. "It's just trying to be really, really, really natural."

This series of the drama focuses on four suspects, a group of childhood friends played by Neil Morrissey, James Fleet and Kevin McNally.

"I didn't know Neil but I've worked with James before years ago, at the RSC, and then he murdered me in a Lewis," Jennings says.

"He drowned me in a pool. And Kevin I've worked with on telly. We did an episode of Inspector Alleyn. We swam naked together at the RAC Club."

As part of the show?

"Oh yes, as part of the show," he confirms. "Scripted. At the RAC Club is an absolutely amazing, incredible swimming pool, and this was set in the 20s or 30s when the members used to get their members out and swim naked."

Jennings may not be playing a prince, but that's a royally good anecdote to end on.

Unforgotten continues on Sunday 22nd July at 9pm on ITV

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