Thanks for all your replies so far...
I'm gonna take a proper, daylight look at the switch today, but from my initial inspection (took the back off and bridged the contacts to complete the circuit last night), it made no difference and still died.
I wasn't thinking about water in the fuel/coils/carb etc as it is running ok in neutral - a little rough but then it's a DT!
So it could run out of gear and stall when in gear because of this, yes?
thanks again