With all the enormous fuss over Sherlock on the telly, David Suchet’s recent retirement from Poirot should not be forgotten. What an incredible innings! The actor finally hung up his patent-leather shoes after a quarter-century of playing the sleuth in 70 stories. The case is not closed for fans, however. Agatha Christie’s Belgian brainbox — to whom a speck of dirt on a cuff is more agonising than a bullet wound — has already returned. He is on stage in the form of Robert Powell, famous for once playing Jesus of Nazareth. The play is Black Coffee, Christie’s first play and the only one with Hercule Poirot in it.

Poirot, a Belgian refugee from the first world war, made his début in her first book, The Mysterious Affair at Styles. Pretty quickly, Poirot’s creator decided he was ‘a detestable, bombastic, tiresome, egocentric little creep’. Her readers didn’t agree and she was lumbered with him. Although Black Coffee was her first play, various theatres had previously adapted her books for the stage and done them badly. They were, by all accounts, mere Cluedo shows, stripped of the real spirit of Poirot or the books. Ms Christie banked the cheques happily enough, but she was never content with these stage rip-offs, so she wrote her own play.

The show premièred in 1930 with Poirot played by a portly actor, Francis L. Sullivan, a friend of Christie who had gone to Stonyhurst, Arthur Conan Doyle’s old school. Today, Black Coffee ticks every whodunnit box. It is set on a sumptuous English country estate, where an inventor, Sir Claud Amory, is the corpse; there’s a stolen secret formula, an amusing foreigner, a whiff of blackmail, all contained on a lavish period set replete with glistening brogues and a proper leather-bound library.

According to the veteran producer Bill Kenwright, the man doggedly behind the splendid Agatha Christie Theatre Company, ‘I had one thought and one thought only for casting Poirot — Bob Powell.’ The man finally agreed — even though he was never a Christie fan — and he now follows in the footsteps of Peter Ustinov, Ian Holm, Albert Finney, Charles Laughton and half a dozen others. Powell spent weeks obsessively researching the character from the books and perfecting his curly moustache. He takes the role every bit as seriously as Suchet, who famously stayed in character even during tea breaks on set. ‘Robert is the latest in a long line. His Poirot is very much his own and he makes you forget anyone else,’ insists Dan Schumann, a producer on the show.

Christie’s popularity has never been in doubt. Her plays and stage adaptations have for decades been the quick fix for many a financially ailing rep. But the revival of Black Coffee raises an old question. Can Dame Agatha write? One sentence in Sparkling Cyanide — ‘Anthony took a gingerly sip of coffee’ — has been cited along with other evidence that she wrote like a blind pig. Has, too, the touristic longevity of The Mousetrap scotched the idea that she is to be taken seriously as a dramatist? Joe Harmston, a self-confessed Christie anorak, director of Black Coffee and all the other Christie plays that the company has staged to date, is dismissive of the criticism.

‘It’s a terrible mistake to underestimate her. She writes whydunnits, not whodunnits. The thing that actors find time and again is that, because of her interest in the psychology of character, she writes terrifically well for performance. This is why we get such good casts. There is something there to get their teeth into, especially for actresses. Christie’s murders are motivated by the need for money. Her father went bankrupt and it gave her a fear of having no money. If it’s not money, it’s passion. It’s about rich people seeking emotional and sexual satisfaction.’

The great achievement of Suchet’s Poirot was to become as much loved by audiences as Jeremy Brett was in all the years that he did The Adventures of Sherlock Holmes for Granada TV. In terms of little grey cells, there is, however, a key difference between the two detectives.

Harmston thinks it works like this: ‘With Sherlock we marvel at his genius. You sit back and think, “God, he’s brilliant,” but we don’t ever think we could have worked it out. With Poirot, the things he notices and the way he solves the crimes are very simple. They are things we have also noticed but haven’t put together in the way he has. The same goes with Marple. The joy of Christie, when it’s well done, is that you shouldn’t at the end think, “Oh, I’d never have got that.” You realise that every clue was right in front of your eyes.’

Harmston also thinks she uses the stage very economically: ‘There’s a wonderful stage direction in The Hollow, for example. A character comes in and sits on the arm of a chair and takes a cigarette out of another person’s mouth and puts it into theirs. At which point, halfway through the first act, you realise they are lovers and it alters everything.’

Is it not true, though, that the plays are basically all the same? Body. Weapon. Clues. Assembly of suspects. Dénouement. ‘No, the joy for me is that they all so totally different!’ says Harmston. ‘People say she’s formulaic. She isn’t. I’d be bored out of my skull by now if she was. Having Poirot in this play means it works in a different way from the other plays where another character has to become a detective. It’s also beautifully constructed. In the end, you do them because you love them, and I am passionate about doing these Christies right.’

If you are not enthused by the prospect of Ridley Scott’s forthcoming remake of Murder on the Orient Express for the big screen, then perhaps Agatha Christie on stage is just the ticket. All you need is a bag of chocolate Revels to enjoy a gratifying world of murder, motive and moustachery in a trusty genre of theatre that, happily, hasn’t been bumped off quite yet.