Av just got back from town in the pouring rain with no roof on but I was still smiling cause av got me self a new bulb. The good fellow at Auto Electrical supplied me with a Lucas LLB643 12V 2.2W 030 for 70p which does the job and I also got an original Lucas indicator for a fiver.
Martha Argonaut - the switch in question is the one with a rubber wheel that runs on the steering shaft just infront of the steering wheel. I see you have a later s2a and don't want you taking your switch apart if it is a different one (I don't actually know if there were any different ones but I thought I'd mention it).
Clivees- I see you are on 666 posts! Have you ceased to cast a shadow or anything wierd?