Thanks - I followed your link and ended up ordering an identical Lucas 35947 on eBay for a Foden!
(The same switch range was also fitted to the MGB GT)
As I had to wait for delivery, out of interest, I part dismantled the faulty switch and gave it a good dose of WD 40 and now it works perfectly! I did the same to the hazard warning switch which was intermittent and now that's good too.
The switch to the left of the hazard warning was also seized and WD 40 did the trick but I cannot identify its function - I think maybe it's pre - wiring for spot lights. ?
The top row of switches all work ok but then they get used a lot more.
Thanks everyone for your help
