I think I'm once again falling foul of my original plan to use a super beetle as a donor of which I have only made use of the engine and pedals and 1 rear axle shaft bearing. Put my FF body on today and lifted in the petrol tank to check it out and there's no way it fits! Is this another super beetle/buggy incompatibility?

I thought this was your chassis? 



Swing axle, and beam. Not 1302/03 Super beetle.

I presume your FF body will be for swing axle/beam chassis. A 1302 has strut front end and is completely different to beam including the fuel tank.

Before you rush out for a new tank, where are you going to have the filler cap?   

Super beetle has a different shaped fuel tank.