AS I browse the bookshop’s travel section a man’s voice drifts across from Crime Fiction. It gets more animated until he lets rip with: “I said to her: ‘If you want 6ft 4ins of seething anger descending on you, Julia, I’m up for it’.”

My wife Clare and I jot down plenty of such potentially incriminating conversations during our weekend in Harrogate, the smart Yorkshire spa town that’s the North’s answer to Cheltenham.

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We are on the trail of crime writer Agatha Christie who was discovered here in 1926 after famously “disappearing”.

She had checked into The Old Swan Hotel under the name of her husband’s mistress after leaving her Berkshire home and abandoning her car near a Surrey lake. Inside, in a scenario worthy of her novels, police found her fur coat, three dresses and two pairs of shoes.

They launched a massive “manhunt” but more than a week passed before a banjo player in the hotel band recognised her.

Publicity stunt? Memory loss? Genuine breakdown? It could be a popular after-dinner debate at the Theakstons Old Peculier Crime Writing Festival opening here on Thursday. Named after the celebrated Yorkshire beer, it was launched in 2003 and has become Europe’s biggest crime-writing festival with, this year, 70 guest authors.
It will be based at The Old Swan, a location for the 1979 film Agatha with Vanessa Redgrave, which stages regular murder-mystery dinners and weekends (it can be murder at The Old Swan, say the adverts).

It’s a grand, ivy-clad Victorian hotel with long creaky corridors, a vast glass-roofed restaurant and a spacious lounge where Miss Marple would enjoy knitting bedsocks and watching intrigues unfold. In the foyer is a display case of crocodile-skin handbags just waiting to be exhibit A. Clare and I are staying in the very room where Agatha, as her fans call her, is reputed to have stayed.

“Actually there’s a bit of a mystery on that one,” manager David Ritson admits.
“During the Second World War a bomb hit the house where we kept our archives so we’re not 100 per cent sure that was the room.” Clare pretends not to hear.

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She is, I must reveal your honour, an obsessive collector of vintage editions of Christie. They occupy an appalling 11ft of shelving in our spare bedroom, enough to accommodate two medium-length bodies end-to-end. Many have covers showing Hercule Poirot in various stages of bald Belgian rotundity. Strangely, Miss Marple rarely features on covers. Another mystery, n’est-ce pas, mes amis? We base our stay on Agatha’s own.

She spent her time in Harrogate shopping and visiting tea shops. The most famous of them, Bettys Café Tea Room, was founded in 1919.
With its corner verandah and potted palms, it’s the town’s classiest place for coffee, lunch and afternoon tea (with First flush Darjeeling, perhaps, and a Swiss chocolate and raspberry torte). The biggest treat is dinner at a window table with the pianist playing As Time Goes By.

Away from tea rooms, Agatha would have felt very much at home at Sissy Jupe, a stylish vintage clothes and memorabilia shop near the town’s theatre. Clare sifts through Festival of Britain headscarves and handkerchiefs and a rail of Fifties and Sixties dresses.

More vintage memories at Pomp and Circumstance, an independent record shop specialising in classical and jazz CDs but also stocking 78s, those notoriously breakable relics of another era of listening. “Want to hear the real voice behind The King’s Speech?” asks owner Peter Robinson, sliding a copy of one of George VI’s Christmas broadcasts from a cardboard sleeve and placing it on a portable gramophone.

He cranks up the machine, lowers the needle on to the record and through the crackle the King’s voice echoes across the decades.
Harrogate, we quickly realise, is full of murderous potential. The Victorian Turkish Baths with their Moroccan arches and richly coloured tiles look just the place for Poirot to lurk in a monogrammed white bathrobe, perspiring fastidiously and exercising only his little grey cells.

The Royal Pump Room Museum serves medicinal “sulphur water” from a natural spring that tastes so rotten-eggs vile you could well imagine you’ve been poisoned. The current exhibition (until September 4) at the Mercer Art Gallery celebrates Victorian artist Atkinson Grimshaw, whose moonlit lanes, spectral houses and foggy city streets seem like perfect backdrops for crime.

Back at the hotel we notice an old lady in a cosy cardigan dining alone and taking in everything going on around her.

Could it be Miss Marple? Could she be here to investigate those Yorkshire crime mysteries Death On The Humber and Murder On The Huddersfi eld Express?
“No,” says Clare firmly. “Those were Poirot cases, not Miss Marple.” Nom d’un nom d’un nom.