Mr Ashcroft's reason for buying his cab, he says, is quite prosaic: "It's convenient, it has got a good turning circle and it is not high-profile."
Perhaps just as importantly for the man bankrolling the Tory party, the black cab is discreet, reassuringly expensive and has the royal seal of approval. The Duke of Edinburgh has been a fan for six years. His latest Metrocab is green in colour as well as composition with a gas-powered engine in keeping with Royal Mews requirements. He is not, however, the first regal enthusiast.
The Crown Prince Tupouto'a of Tonga was said to have three taxis, decked out in white leather, thick carpets and cocktail cabinets.
Equally decadent was Liberace's snow-white cab customised with gold fittings.
Simon Hughes, MP for Southwark North and Bermondsey and a contender for leadership of the Liberal Democrats, possesses the most eye-catching version, a luminous orange machine.
The actor Stephen Fry was said to be "very upset" when vandals attacked his prized P-reg version in February. The Governor of the Falklands, Richard Ralph, reluctantly gave up his maroon cab two years ago after mechanical failure nearly dumped him in a ditch.
Keith White, chairman of the Vintage Taxi Association, which has 300 members in places as far-flung as Barbados, believes the attraction is simple: "They are full of character and quite unique, the only purpose- built taxi in the world."
Stewart Pessok, who has driven a taxi for 30 years, and now edits the Taxi newspaper, has no illusions about them. "It is as London as London can be. That's the only thing which really attracts people. They are just old, smoky, diesel-driven cabs," he explained.
London's first motorised taxis - a dozen electrical cabs which reached speeds of 9 mph - began operating in the West End in August 1897. The latest Metrocab, launched in October 1995, costs between pounds 26,000 and pounds 31,000, though fans of the classic design - dating back four decades - can pay as little as pounds 1,000 for a second-hand version.