I found that one of the hinges on the open/vent flap had come off. The Torx/star headed screws had worked loose and popped out. By holding the hinge in place it was possible to open the flap and replace the screws. Most screwdriver kits now include the Torx screwdriver bits.
Having checked the other windows, I found they were all about to fall off so retightened them all.
Pete
August 2015