To avoid paying for a 3rd Mot, I fitted a separate switch anyway. Since cutting lumber support, no more fuse blowing .

Next time I have the seat out, I will reconnect original seat base wiring (removing my switch)

Petrol flap opened correctly. Mot passed.
Before Mot, I asked "if flap won't open, can I remove it?".

In the opinion of my Mot tester "no, unless you fit a locking filler cap".

I think this may be a misunderstanding of the testers handbook that talks about a cap locking in place. Suspect the handbook means the cap must lock in place so you know it is fully closed (unlike cap that keeps spinning without locking)

All done guys. Thanks for the info, support and encouragement.

Some more posts coming for seat belt catches and reversing sensors.

Cheers!