There is a magnetic switch on the fuel cap door that sometimes fails which stops the passenger door working. The switch is designed to stop a passenger getting out of the car during re-fuelling as the door could open onto the fuel nozzle.
Looking up from underneath the wing after removing the fibre splashguard, the switch is mounted on the side of the fuel inlet and flap assembly
If this switch malfunctions (can be caused during winter if frozen water cracks the switch body) then the passenger door will not operate. Moving the magnet closer to the reed switch may be all that is necessary if the switch is not broken.
operation of the switch can be checked by lifting the round flap under the nearside rear seat (seat fully back)
when the flap is lifted, remove the circular cover and you will see the top of the fuel tank and a two-way connector leading from the fuel filler flap reed switch to the door ECU