It's alive, it's alive! Points and condenser. Running like a charm. Very busy last two days and got back to it this morning. I checked the points gap and it was good, however the points seemed to have an arcing mark on them (pic in post #18). In the end I believe it was a bad condenser, but because I changed them both at the same time I can't be guaranteed. I think the key info was the backfire symptom, a tell tale sign I have unexploded gas leaving the cylinder (intermittent spark).
Thanks to all for your help.