I think there is a web site dedicated to finding out his true identity. I typed into my search engine "who is the Stig?". It came up with a blokes name and I still had never even heard of him, apparently he's some sort of professional test driver.
His name is Perry McCarthy, a man dubbed by the Times newspaper the unluckiest racing driver in the World. He got in on the F1 circuit, tried to get in on 8 races but failed to qualify 8 times.
The link didn't work as well as I thought it would, but does take you to the right place. On the page the link takes you to, go to 'more from this section' on the top right of the page, click on celebrity Essex, then click on Perry Macarthy.
There's been two stigs. I dont know whether they were the one and same person just in a different racing suit though The first Stig was killed off after driving a stripped off XJS of an aircraft carrier.