Well, if the certificate does have any value (not the sort of thing I have ever thought about), having the wrong number just makes it a piece of paper.
Yep, I never had an problem with PMT, although I have never had to do a return, and it is only in returns do you know how good someone is. I have done G4M, Andertons and Bax returns and never had any problem with andertons and G4M.