I've disassembled and cleaned the whole carb, and all was well.
I've now finally found the right plugs (Champion L90C = original KLGF50), which are much hotter, which will reduce carbonising.
Also, correct jets (115 main and 210 air corrector), so that should all help.

I do have one 'odd' bolt which screws into the underside of the choke chamber, over a very small hole into the chamber. It is non standard and seems to blank it, but can't find what it should do.

Can't find a supplier for the correct choke which should be 30-21, mine has a 30-23 in. Any thoughts?

Cheers