Well a little insider information....
My dad was at silverstone a few years back... Possibly 3-4 now.
During a chat with Murray Walker he let slip that the Stig was someone who was on the Williams paybooks at the time. I was led to believe that the driver was something of a youngster and possibly a test driver.
So that was during my dads visit with Williams to the British GP.
He worked for allianz at the time which along with BMW was the main sponsor.
Hope this helps track the mystery man down.