Ok I'm totally confused now. Checked the timing, 28 degrees. Checked the diaphragm in the carb, fine. Cleaned the plug, sparks fine. Filled up with fuel with the new synthetic oil mixture. Choked on until it popped. Then choke off, three flicks and perfect run with no backfiring. I don't get it! Completely back to normal with good idle and throttling. Holds max revs no problem. The only thing different is I ran it without the spinner and backplate and I wonder if we weren't getting the prop nut tight enough before. Because number one son had already suffered one injury from a backfire (bad bruising only fortunately despite wearing a heavy duty leather glove) maybe we weren't giving it enough of a flick (more worried about getting the hand out the way than anything else) and this was why it was tending to backfire. Definitely much more oomph with the flicks this time.

Anyway seems sorted now but I still don't get why it started backfiring?