I contacted a specialist repair site as well. They suggested the diaphragm pump, so I bought one, a sod to fit, but got it installed. It ran for over 2.5 hours yesterday whilst I was changing the steering box on my LDV. Stopped it, moved it into the garden where I could watch it (or at least other people couldn't nick it!) and it managed 20 minutes at most! Arggghhhh!!!
Plenty of fuel in the carb. I'll have to investigate spark or the stepper motor on the butterfly. Tempted to remove it and leave it at a fixed setting and see what happens...