I thought that was pretty widely known or at least generally believed to be the case.
I think it likely that there have been other "Stig" test drivers along the way though (apart from the obvious such as setup stuff like standing at bus stops, F1 car etc tests and of course McCarthy's era), as body shape hasn't always been consistent.
It used to crack me up that another show from England had both Collins and McCarthy on it as drivers. I think it was/is called Vroom Vroom or something like that (I haven't seen it for a while). It used to have, as one of its regular features, a cat and mouse chase through a car park with an old Jag as the cat.
McCarthy used to race a business person around a track in that persons work car with the other driver using a 350Z - eg hearse, ice cream truck etc
Collins did tests of one car against another eg an Ariel V an AMG, that sort of thing.
I'd kind of like it to be wrong though - Collins playing jokes or whatever. I think it works better with the seed of doubt and therefore some mystery. More fun.
One of the cleverer things they have done with that show.
Tim.
