There is a unit in the boot.
Push down the carpet and you will see.
The part name is: Fuel Filler Latch Box. This is the wrong part.
There should be a switch to the left hand side around the hinges. white button
When you press it your filler flap should be relased.
If you take apart the Fuel Filler Latch Box with a screwdriver you will find an electro magnet (this is the lock)
and a electronics (PCB). Usually the electronics not operating (because old and wrong).
There are two ways for this problem.
-You can buy new electronics
www.all4jags.com(The installation is not to difficult if you have screwdriver and a soldering iron).
-Or a complete Fuel Filler Latch Box
www.jagbits.com www.jaggraveyard.com