Thanks for suggestions to my earlier post.
Concluded that it was a spark problem. Carb was fine with all jets clear and diaphragm ok, so no problem there. Reset plug gaps to 25 thou (from 30) and it runs fine.
I think I need a better coil.

"What right twice in one lifetime? When your hot your hot!" (red dwarf quote)

My first car had dodgy leads, full throttle was so bad it behaved like a kangeroo.

Try changing the leads first, then the coil.
Darren