Ok, I've managed to remove the flap, so for future reference here are my findings.
The flap is held in place by one star-headed (security type) screw which can be clearly seen with the fuel flap open (or missing in my case). One the screw is removed, the next step is to remove the filler cap, then carefully pull the rubber away from the neck of the fuel filler pipe (the bit that you put fuel in at the petrol garage).
Next, you can then start to tease the unit out towards you. The small drainage pipe at the bottom of the filler recess isn;t secured anywhere and just pushes down intoa large hole under the card - this can be clearly seen once the filler unit is removed.
Next, we're trying to remove the what remains of the hinge mechanism from the surrounding unit so we can replace just the filler flap.
More updates to follow.