As much as they pretend otherwise, troubled TV executives turn to detective series when they need to boost ratings.

A buccaneering but flawed crime solver, railing against bureaucracy and corruption, usually gets producers out of a hole.

Robert Powell, who made his name as Christ in 1970s series Jesus Of Nazareth and later played hapless TV detective David Briggs in The Detectives, says the format is irresistible.

“I think detective fiction is and always has been loved by audiences because they quite like being thrilled, they quite like being left guessing and they quite like trying to outdo the detective themselves.”

Yet, after so long in the business – he’ll be 70 in June and has been acting since the 1960s – he is still amazed that time and again we want more.

“No matter how many times television execs try to get away from it, it always comes back.”

That’s because it’s a good template.

“The fact it’s a police series is neither here nor there. The story is what counts. You could set the story anywhere, with a hero who did anything.

So why not make him the detective?”

His latest role is playing arguably the sharpest sleuth of the lot, the Belgian Hercule Poirot.

David Suchet’s take on Agatha Christie’s detective ended its television life last November in Curtain: Poirot’s Last Case.

Suchet starred as the detective from 1989 and completed the catalogue of 33 novels and more than 50 short stories.

Yet Powell believes it’s easy to stay with a character too long, turning the characters into caricatures. It’s why he walked away from The Detectives after five series.

“We were asked to stay but we thought it would diminish it. We didn’t think we could add anything.”

Benedict Cumberbatch as Sherlock Holmes should take note.

“Sherlock is terrific, but it has to be awfully careful it doesn’t disappear up its own bottom.

“It’s just starting to get a little bit too clever.

And abandoning and forgetting the story... they are very important and the other characters are very important. There is an inevitability if you stay with anything too long.”

Though he confesses he is not a fan of Doctor Who, he says that show has got the format right.

“It’s a good idea to change the persona of the doctor every two years, otherwise it becomes too predictable and too easy.”

Avoiding predictability is something Powell is constantly battling. It’s why he took the part in a new touring production of Black Coffee.

“I say yes to parts based on script and character and if it’s different from something I’ve done before.

“The trouble is as you go on, the harder it is to find something that is novel, new and different.

“Playing Poirot struck me as a nice challenge and a bit of fun.”

Fun is the adjective he uses most to describe his version.

“Our Poirot is a little more rounded. He has all the qualities of the TV one, but he also has a sense of humour.

“I think he is slightly warmer, but working in the theatre that would be inevitable in an attempt to make him more likable.”

It contrasts with Christie’s opinion of her creation fuelled, in part, by the fact he’d taken over her life.

“She hated him. She found him detestable, pompous, egotistical, self-opinionated; she called him a little creep.”

Her one stage play, Black Coffee, is set on an English country estate which is thrown into chaos following the murder of eccentric inventor Sir Claud Amory.

Powell calls it a tough role.

“If he is on stage he is completely alert which is terrific but tiring.”