There is writing on the switch above the 01203Q. Can you show it so it is legible?
Does eBay have any spare or repair models?
Is it 100% the switch and not bad contact with the battery? If you squeeze and it does not work, the instinct is to shake it, and that might make a dodgy battery contact re-connect?
Theres 7 wires going to the switch which tells me it is not a simple on off, or variable resistor type. It will have a load of resistors and pcb traces for the switch to wipe, giving the impression of variable speed, but really its speed steps.
Quite simple inside, a few springs, I have had a few apart before and can be cleaned up.
I'll try and open up for a look but if not I'll probably just buy another bare unit and keep this for spares. Too much hassle getting one from the US. Refurbished ones on eBay are about £25.