I’d suggest running it until it is hot and stalls and then immediately checking if you have any fuel in the carburettor bowl (taking all necessary care not to get any fuel in contact with anything hot!). If there’s none, then fuel will almost certainly be your problem.
The heat from the engine can be enough to cause evaporation problems, and the fuel pumps (one way valves, diaphragms etc.) can fail quite slowly with the pump getting less effective as time goes on. I’ve recently had similar problems with my 16 – which now has an electric pump which is so much better.
This is now sorted thanks…..the tappets were tight resulting in the engine stopping when it warmed up. I should have adjusted the tappets 500 miles after the engine rebuild (now done 1200) but its such a pain to have to take the exhaust off to do it that I didn’t get round to it. As far as I can feel from the starting handle there is still good compression so no long term harm done.