Diana herself points to the artificiality of her first film for The Mrs Bradley Mysteries: "This is a long way from the gritty realism of something like Prime Suspect," she concedes. And so it should be. Diana's move into Mrs Marple territory is designed to slip smoothly into occasional prime slots such as the August Bank Holiday, where a diverting but undemanding period whodunnit is just the ticket.
ITV experimented earlier in the summer with a rather nasty serial killer drama in one of these holiday slots and the lowly ratings should deter them from any repetition. People may drop dead around Mrs Bradley but the tone never gets too terribly serious . And if Diana's detective proves popular there's plenty of scope for more adventures.
The character comes from the novels of Gladys Mitchell, who wrote 41 of them. In her day Miss Mitchell was allegedly as popular as Agatha Christie and was publishing novels from 1929 to 1979, four years before her death, but it has taken until now for TV to latch on to this untapped resource.
Maybe Mrs Bradley was just too far ahead of her time. Our elegant heroine led a rather shocking life by 1920s standards and tends to spout worldly one-liners which are far from politically correct.
After marrying and producing a son, Adela fled from her worthy but terminally tiresome husband ("Marriage is one of those things it's best to get over and done with quickly, like chicken pox") because she was in imminent danger - of being bored to death. Waspish asides delivered straight to camera (a la Francis Urquhart in House of Cards or, for that matter, Rab C Nesbitt) add to the feeling of a modern set of attitudes trapped in a creaky 20s plot.
Says Diana: "If she were just another female detective I don't think I would have been interested, but she's such fun to play, and I've never played anything similar... Mrs Bradley is very liberated and unconventional."
With the much-criticised movie version of The Avengers now on release and videos of the old 60s shows back in the shops, thoughts inevitably stray to the liberated, leather-clad and unconventional Mrs Peel. Diana laughs any comparisons out of court:"It' s not remotely like The Avengers. Emma Peel wasn't really a detective, she was Steed's right-hand man - or rather woman. She did all the physical stuff and I don't think I'd be up to that any more. It was 30-odd years ago - although people still talk abo ut it and I don't mind that, because it was wonderfully helpful to my career at the time."
Yet there is one element of The Mrs Bradley Mysteries that sticks surprisingly closely to the on-screen chemistry that helped make The Avengers such a hit - Mrs B's relationship with her "trusted confidant and handsome chauffeur," George Moody. NeilDudg eon, looking a little overweight for that "handsome" tag, plays George and there are constant hints that there's more to his relationship with m'lady than meets the eye. As with Steed and Mrs Peel, you are meant to wonder just how far their intimacygoes . Says Neil: "It's never said, but people might think 'Are they? They're not! They are? They must be!'. It'll leave people guessing, because if you came right out and said they are, you are into a whole other situation with their relationship, and Ithin k it's more interesting if you keep things more mysterious."
The difference is that George is dependent on the older, richer Mrs Bradley, and though she may rely on his support Mrs Bradley is dependent on nothing and no one, which adds to the contemporary edge of the costume-rich period piece. She is a psychoanaly st as well as a detective but she doesn't need to work to maintain her lifestyle: "She is a sleuth but she isn't really employed by people," says Diana. "She's got pots of money. I think she probably makes it by being a whizz on the stock exchange."Dian a herself has not reached that stage but she does tend to accept work only if it interests her, even if that means going without: "I was out of work for 22 months after winning a BAFTA for Mother Love," she admits.
"All actors go though quiet periods, but not many of them talk about it." Mrs Bradley would admire her candour.
The Mrs Bradley Mysteries, Bank Holiday Monday, August 31, 8.30pm, BBC1.