Martin,
The top handle on the gear selector is held in place with a small allen key-type grub screw.
Look at the back of the handle(the side that is furthest away from you as you sit in the car), you should find a small hole about 1/*" dia.
From memory the allen key you will need is about 2-3mm. Screw out the grub screw enough to release the top bit from the lever and lift off.
Pry out the wood or leatherette covered cover(its usually held in with clips) and that will take you fasr enough into the selector to check if the selector rod from lever to gearbox has come adrift. It may also have come adrift at the gearbox end - need to get under to check, but be careful going under a car, if only using the jack!!!!!!!