Hardly a secret though was it, really? The BBC used a bunch of drivers for the 'Stig' bits. The whole point was to keep the Stig as an enigma, as paying the drivers extra appearance money would have cost loads, and had he potential issue of a driver's ego getting too much for the other presenters. Plus, racing drivers can get injured or killed, and their race calendar may have clashed with the shooting schedule, necessitating a pool of drivers they could use. The Stig thing conveniently covered this up.
Watch a bunch of episodes from different series. Notice how 'the Stig' is different heights compared to Clarkson….