Try pressing the central locking button on the driver door handle (Red key symbol) at the same time as pressing the fuel flap release. This works for me too open mine.
I have the same problem as you, I have replaced the fuel flap release button on the B pillar with a new one and the problem persists. I have also bought a new solenoid for the flap but after testing mine on a 12v supply it works fine. Looks like my problem is the drivers door micro switch. It seems they suffer from dry cracked solder on the electric side of the mech due to lead free solder that is so abundant in early 2000's electronics.
The way to test if you microswitch is faulty I believe is to listen out for the central locking cycling, lock,unlock, when you turn the key.