I'm curious about this please post your findings.
I got into mine this evening, and have found the following:
The bellows for the filler flap lock is a large bellow of approx 2" (5cm) diameter, which pushes out on the red plastic arm.
The end of this red plastic arm is the flat red plastic plate, through which a metal shaft is fitted.
The metal shaft then goes back past the bellows assembly, through an airtight seal (which is the cause of all the resistance) and out to act as the locking element for the flap.
Not sure how I'm going to ease the airtight seal, without compromising the rubber; silicone grease (or WD40) would seem to be an option, and avoiding mineral grease.
Reading your comment about the two plungers should have rung the bell louder.
So from your description, it would appear your activating plunger has separated from your locking bar, and so you don't have a working lock.
I have also bought a vacuum tester, and have been able to confirm that all the locking circuits around the car work (suck to lock, puff to unlock) so all I need is to get a working PSE. Note a vacuum tester can only have 1 Atm pressure, but be careful with your pressure source; more than 1 Atm (14 psi) and you may damage the system. Do not use an air-line!
Three "circuits"; filler flap has its own line, drivers door and central compartment has one, and passenger door and boot have the third. Probably based on the length of line and the volume of air required to activate the remote bellows.