Hayter 13/30 Lawn Tractor - cuts out?

Hayter 13/30 Lawn Tractor, very good condition - Ran it last week, cutting two complete trips around garden.

After end of second trip, it cut out, so I left it ten minutes then topped it up with fresh fuel.

Tried to start it again, would not fire. Put throttle to the 'choke' ( fast ) position, it fired up, moved throttle to run position....cut out !

Did the same thing again, choke, fires, change throttle...cuts out.

Fuel good, fuel lines clear, stripped carb, all clean, air filter clear etc.

Plug looks ok.

Any idea's ??? - could it be plug or coil - or anything else.

Thanks

Alastair
August 2014