While fault-finding my non-operational fuel gauge I removed the tank pump and sender assembly (what a bitch to work that through) and found the plastic float ball had a tiny hole in it where the float wire arm clipped onto the ball. The float had filled with petrol and so wouldn't float.

After draining the float ball I sealed the hole with a petrol resistant gasket sealing paste and reinstalled the pump/sender assembly. (This was even more difficult than the removal and eventually required me to remove the fuel tank to get it back in.)

After about a week passed and next refilling the tank my gauge read zero again. I suspect the ball has filled again and will need a different type of repair.
Has anybody tried to repair the float ball (I think it is made from clear polythene) or replace it by some alternative float material (a table tennis ball maybe?).
Otherwise what type of adhesive will permanently stick to polythene plastic and not be affected by petrol so that I can seal the hole? The hole is only about 2mm dia. and is in the groove where the wire sender arm clips in.