Johnno,
It does sound as tho' your switch has broken. However, an easy job this one - Use a small screwdriver, instrument type or similar to poke in the little hole in the black, tee-shaped switch simultaneously pull the switch and it will come off in your hand. This reveals a chrome, round nut with two cut-outs at 180° apart - use the small screwdriver to turn this nut anti-clockwise and unscrew it completely. The switch will now push through the dashboard and may be withdrawn and dropped down behind the switch panel. You may have to remove the trim panel which forms the roof of the storage space above the radio position. This pulls down from the front, held by two black plastic push-pins and can be pulled towards you to release from the two metal clips that hold the front edge (or back as you look at it). Pull off one wire at a time and note its position or label it with a piece of masking tape. You should then have the switch in your hand. Repair or replace and reverse the above for refitting, as they say in all the best manuals.
Hope this helps.
Regards, John.