Thought I would give an update on this problem.
I put a spoonful of petrol in the air intake. It fired, spluttered, but kept going. I expected it to stop after it had used up this small amount of fuel but it kept going.
It has worked fine ever since. I guess the petrol in the carb over the Winter evaporated, left some nasties behind. By injecting a little fuel and starting it blow the nasties out.