[Darren69]

Does the gasket swell after use - or when removed - and I need a new one? It looks undamaged, so I thought I'd just re-use it - but that might explain it...

I've not had to do one of these before but I've read horror stories of those with less patience actually denting the tank trying to force these back in, so its my guess that they swell and need replacing. The large o ring on the bottom of the 748/916 fuel tank that seals the fuel pump flange to the bottom of the tank cannot be reused as its too big and gets trapped however careful you are and a new one goes on so easy its mad. So that would be my guess.