Craig and GraemeS,
Thanks again for your responses, I've removed the stop light switch and ordered another. The dismantling instructions from my manual on CD clearly don't cover the later L322 model, everything is different. On the plus side the LCM and the surrounding area are bone dry with no evidence of water ingress. The clip which I think locks the plug into the module slides down but does not seem to release it from the module still that's a job for tomorrow. The switch was marked BMW and was a pig to get at. Ideally I would like to have removed the air bag panel to get better access but I need to look for help on the correct technique.
I am reading reviews on the diagmostic units and agree that I will need to buy one.