If you undo the carpet on the right side of the boot, look forward (you might have to remove the polystrene block over the locking pump unit) you will see the actuator for the flap, there is a rod going from tha actuator, if you give it a pull the flap will open.
If the rest of the central locking system works OK the flap actuator might of failed or it could be the vacuum pipe has fallen off/split
nigel, May 2011