“I found out years later why they chose me,” said Jean. “It was because in the section for ‘hobbies’ on my joining form, I’d written down ‘doing crosswords’. ”

If it sounds a terribly English method of finding the “right sort” for the job, the method suited not only the time but also the place; the former country mansion where wooden sheds had been hastily erected in the grounds to house the dozens of young chess players, mathematicians, students, soldiers, science grads and puzzle-lovers whose code-breaking work was later claimed to have shortened the war by two years.

When I joined a private tour of Bletchley, I found that the sheds were still there, rescued from demolition during the 1980s by enthusiasts. So too was Jean, who at 79 now guides groups of visitors briskly around the site, livening her tour chat with gossipy information: “they rigged up chutes to pass messages between the huts, using filing trays tied with string. You’d let the next-door hut know there was something to pass to them by banging on the chute door with a broom handle, so they’d open it. It meant you didn’t ruin your hair-do by having to go outside in the rain.”

Jean worked for two years on Alan Turing’s Bombe machines: vast electro-mechanical constructions of code wheels which each day had to be wired in different sequences by the Wrens to break the Enigma machine-encoded transmissions.

A Bombe machine is being rebuilt now at Bletchley, as is the Colossus, the world’s first computer. A slight, bespectacled chap was working on the room-size machine as our group swept in with Peter Gamble, our afternoon guide and an ex-Navy cryptographer. He turned out to be Tony Sale, champion fencer, motor-racing driver, founder of the Computer Conservation Society, lecturer and all-round hero to anyone interested in codes, ciphers, computers — and Bletchley.

He and his wife Margaret, along with others, have poured their savings into the trust to save the park from demolition. The men in our group nodded in that blokeish way as Sale launched into details about vacuum photocells, valves and paper tape-reading systems while I wondered how the Wrens kept their cool in the stifling Colossus rooms before air-conditioning (and deodorants), and noted that the fabulous model submarine used in Mick Jagger’s movie, Enigma, about the station ’s work, was slowly peeling as it stood outside in the rain.

Bletchley is peculiarly sited; I’d expected to find it hidden deep in a green hollow, miles from the public view. But perhaps it’s true that the best place to hide a secret is in full view of everyone — the mansion and huts sit at the end of a cul-de-sac of brick villas in the prosaic outskirts of today’s Milton Keynes.

This setting, with its delightfully amateurish air of wartime make-do-and-mend, made a stark contrast to the second part of our tour — Blenheim Palace, with its magnificence, pomp and circumstance.

We drove through Woodstock’s mellow stone streets, turned a corner, passed under a stone gateway, and were suddenly confronted by a quintessential English scene — the massive, opulent stately home sitting in Capability Brown’s artfully contrived parkland.

We were ushered in for a private, early-morning tour of the state apartments: sumptuous hangings, porcelain, paintings, gilded gewgaws and fabulously ornate furnishings. I escaped to wander the parterres, the water gardens and the arboretum in the pristine, manicured grounds, passing the Temple of Diana, where Winston Churchill proposed to Clemmie.

The whole experience was absolutely English; a group of like-minded people joining a coach tour to see places they all knew about but had never quite managed to get to see. And don’t knock the coach bit either — it saves endless rows over map-reading.

NEED TO KNOW

Getting there: Bernice Davison joined a group with Heritage Touring (01300 321485, www.heritagetouring.co.uk) to Bletchley Park and Blenheim Palace, staying at the Bear Hotel in Woodstock. The next tour is April 10-12 and costs £339 per person, including two nights’ dinner, B&B, welcome reception, private tour of Bletchley Park including buffet lunch, private early tour of Blenheim Palace state rooms, professional guide and coach travel from the hotel.

Times recommends

• The smart guide to travel blogging
• 30 top takes on New York
• Europe's 20 best Christmas markets 2009