Fantastic Hartnell impression - I'd have believed it was his voice. Somebody send John Guilor to Big Finish immediately.
It's strange, some people are utterly convinced, but others including myself have never mistaken it for Hartnell for a second.
Personally I thought Jonathan Redwood did a better Hartnell for Altered Vistas Desert Island Discs recon, but I say that purely from memory.
If Big Finish ever start using substitute Doctors in such a manor its the day I stop buying.