My fix rectified the same fault without re-wiring the sender unit (if only my fault-finding had been as quick

)
1. Drill off head of external rivet (photo)
2. Carefully lever plastic terminal block off rivet shank and sender flange.
3. Remove both metal terminals from plastic terminal block (photo) and throughly clean them and external mating surface of sender unit flange with wire wool
4. Remove internal rubber insulation washer (and retain), cut off redundant shank of rivet and drill out the internal, large head of the rivet (which has the rheostat wire soldered to it) so as to make it into a washer.
5. Re-assemble using 4BA (or metric equivalent) brass nut, shake-proof washer and bolt in place of the original rivet making sure that the rubber insulation washer is replaced correctly (so as to insulate the rheostat wire from the sender flange) and that the plastic lip of the terminal block is correctly postioned in the hole for the same reason.