If evolution is about adapting to one's environment, there can be few species of motorised transport more highly evolved than the London taxi. Conceived in current FX-4 guise, in the early Sixties, it is one of only two vehicles - the Routemaster bus being the other - to have been engineered to meet the specific demands of the capital's traffic-clogged streets.
By law, a taxi must have a turning circle of less than 25ft - better than a Fiesta - yet it is capable of carrying six people, driver included, in relative comfort. The high roof-line and wide door apertures make entry and exit easy; certain versions can also accommodate a wheelchair.
The large, torquey diesel engines may be short on refinement, but they are good for more than 300,000 miles without major attention. Performance is modest, but adequate in town; more important is the fuel economy which, at 25mpg, is impressive for a six-seater in stop-start traffic.
A brand-new FX-4 Fairway, the current Nissan 2.7-litre engined black cab, will set you back about pounds 22,000. But it is possible to pick up either an mid-Eighties FX-4S or a fully restored Austin original from the late Seventies, for around pounds 5,000.
The original Austin-engined taxi is seen by many as the definitive model. But drive one before buying, as the steering lacks power assistance and many will find it too heavy. The first Land Rover-engined model, the FX-4R, used an underpowered 2.2-litre engine which overfuels, averaging less than 20mpg.
Far better is the 2.5-litre FX-4S, or the 1988 Plus version, which gained a more spacious interior and the ability to carry a wheelchair. The current Fairway model's 2.7-litre Nissan engine is the most sprightly, but parts prices are higher. A tidy FX-4S can be yours from pounds 5,000; the cheapest Fairways come in at around pounds 7,500.
Taxis are put out to pasture for one of two reasons: either their condition deteriorates to the point where the Public Carriage Office refuses to issue a Hackney Carriage plate or, given the increasingly intense competition from unlicensed minicabs, a driver decides to give up the game.
A taxi which is ineligible for a plate may still be roadworthy, since even expertly repaired accident damage to the chassis can end a cab's professional career. But structural rust is the main reason why a vehicle is given early retirement, and the symptoms - particularly corrosion to the inner sills - are often overlooked by the casual viewer.
For this reason, it is wise to invest around pounds 100 in an independent AA or RAC inspection before buying one of the handful of unplated taxis regularly advertised in Exchange & Mart and Loot. Recently plated cabs sold because of financial distress, or a fully restored vehicle, may be better buys.
London Cab Sales, based in a turn-of-the-century warehouse in Brixton, leads the drive to restore old taxis. The company, which in rosier times operated many thousands of licensed taxis, now concentrates on spares, repairs and the sale of painstakingly rebuilt vehicles, often to foreign collectors.
Director Tom Bunn explains that the FX-4's old-fashioned separate chassis means that rusted body panels, such as the vulnerable wings, are often filled and resprayed by owner-operators, and need to be replaced. The inner and outer sills may need renewing, and the interior a re-trim.
The unstressed engine may demand a rebore, but provided the previous owner has kept to the 4,500-mile oil change service schedule, one unit can last for 20 years.
Reconditioned mechanicals are cheap: pounds 400 buys an automatic gearbox; pounds 600 an engine. Bunn estimates there are over 100 independent taxi repairers in the M25 area, so keeping one on the road isn't a problem.
The London Cab Sales workshop is dotted with taxis in varying conditions and states of dismemberment. Bunn explains that vehicles with unsalvageable chassis but good panels and mechanicals are cannibalised to put less damaged examples back on the road. The restoration process also allows buyers some flexibility in tailoring a taxi to their own specification: the lethargic engine of the 1982-85 FX-4R can be upgraded to the later 2.5-litre if a suitable unit is available.
Bunn's ethos is similar to that of Charles Ware, founder of the Morris Minor Centre in Bath and guru of the durable car philosophy.
Ware challenges the motor industry's view that a car has a finite life, after which it must be replaced: with a combination of preventative maintenance, once-a-decade restoration and judicious mechanical upgrades, a Morris Minor - or London taxi - can remain reliable, roadworthy and driveable for as long as its owner wishes.
Many of London Cab Sales' restored vehicles are shipped to Japan: to Tokyo businessmen, the London taxi is a status symbol as well as a novel way to beat traffic in one city whose congestion is worse than our own. Others are bought for old people who value the ease of entry offered by the high roof-line and wide doors.
Some also go to senior executives in London's merchant banks, who find them the quickest way to get around the City. Do they use the bus lanes - fair game for plated taxis, but strictly out of bounds to the
private owners of old black cabs? 'It's possible,' admits Bunn, a twinkle in his eye.