At an auction selling hundreds of ex-police cars, Quentin Wilson reveals that they can make a surprisingly good buy- and they're cheap.too. They only come in white, and most have covered globe-spanning mileages, but they are always well looked after and at giveaway prices. Top Gear's used-car guru even finds some bargains in discreet metallics from the plain-clothes branch. Meanwhile, in a deserted airfield, Tiff Needell test-drives a new British sports car, the Noble M10. With a 2.5-litre engine mounted behind the driver, the Noble gives near supercar performance.