Bad news - mower was dragged out last weekend having done last year successfully but now would not start.
Again I pulled the carb apart and blew out the jets. On reassembly it started first pull but only ran for some 15 seconds and then stopped. It will restart a few minutes later, on choke, and run for another 15 seconds and stop.

It seems the filler cap vent solution was a red herring.

I am convinced it is something to do with the vacuum control on the throttle rather than fuel delivery....

Anyone got any thoughts on this?