Update: The breakdown service managed to get the alternator belt replaced on Tuesday evening using the old belt I mentioned from the boot as a temporary stopgap.
I'm driving the vehicle to the garage tomorrow where I'll ask them to see if there's an underlying cause for the belt to come off (other than an old belt). I reckon I'll have problems getting hold of the OEM belts at short notice, but I may come back myself later and replace the belts with OEM ones
Fingers crossed that between the meths and the replaced sensor the car starts first time tomorrow morning