If i recall correctly, inside the boot, on the right hand side, above the accessory socket is a flap, open the flap and there should be a cable release for the fuel flap - just pull it....
I have a similair problem (I think). Take key out and press the button several times fairly fast it opens on the third or fourth press (well does for me).
The circuit for the flap release is via the driver's door lock. If this is faulty
then the flap release will not operate - same lock also controls the tailgate
mechanismn. I assume the flap mechanismn is free to operate.