After dismantling the switch and giving it all a thorough clean up and check over it I've found the electrical half of the switch is just fine - all contacts clean, unpitted and operating beautifully. The problem is very limited movement in the bellows part - I wonder if the gas has leaked out or something. However, having cleaned it all up and fixed the broken brass collar, it seems a shame not to leave it there looking nice, and anyway I need something to fill the hole in the radiator. So I'll refit it and add a hidden Kenlowe switch to actually operate the fan. Thanks again for the links.