Ok then, a Christmas update.
This seems to be a carb problem.
I took the carb apart, blasted some compressed air around it (again) and put it back together again. In reality, I don't think it was blocked in any way, but did it for good measure.
After that, it seems to be working ok again now.
Then I rode it 45 miles twice (visit to parents) and now it seems to be running ok, if a little hard to start. In fact, it's now running faster than ever.
I'm sure it will fail again - at the very least I don't think the carb is property tuned - but at least I know it has something to with the carb in one way or another.
Thanks everyone for your help with this. I seem at the moment to have got there.
Matty