My plan now is to change the plumbing to three line and replace the QuikFill Valve with a decent Fuel Dot. Therefore the fuel from the tank goes direct to the carb. Eliminating long lines and possible air leaks. See what happens then.
I have ordered a new CM6 plug and if necessary after replumbing and still no joy, I will try a spare ign module although I still think it is a fuel issue, but just covering all bases.