Thanks bookworm, the theory about Vmca had crossed my mind as well...but the AFM I have is from 1963 which is presumably before the FAA changed the Vmc figure from 80mph to 90mph. It may be that the manual I have is in fact a mix of stuff from different dates...it says 1963 on it but I know the plane is a 'B' model built in 1966 which is a bit bizarre, although it does have the plane's registration on it. So who knows about that?
The unofficial Owner's Handbook for the PA30B (well worth getting a copy, Essco sell them) quotes take-off runs based on a lift off speed of 80 mph with flap 15, with an assumed speed of 91 mph at 50 ft.
Actually I ordered one a few days ago...I'm glad you think it's worth getting. Although of course it is 'unofficial' it is still a Piper publication isn't it?