Fuel System Filler Cap
Stiff filler cap
(penned by Covkid):
This is basically a cleaning/disassembly. After a time, filler caps can be stiff to turn - and to avoid key being snapped off in the lock!.
If you remove the filler cap and flip it over, you'll see a small pin through the red plastic housing.
Push this through and the whole assembly comes apart.
The last part is a steel washer which invariably gets corroded, hampering the movement of the filler cap when you turn it.
Remove all the rust and coat lightly with grease (ideally silicone grease if you have it) on the underside (not rubber side). Also put a dab of grease under metal plate which the red cap sits in then reassemble
If the rubber washer has gone, it wouldn't be too big a deal to cut some out of a sheet of rubber. Mine was fine.