Thank you, Bob! I'm still a rider not a tinkerer but she's getting close.
Another way to check and set your mix screws is the "ride and adjust" method. Take a small screwdriver along with you and go for a ride. Once the bike is nice and warmed up, pull over and open each mix screw 1/4 turn. Ride some more and note the effects. Popping on decel going away? Pull over and give the screws another 1/4 turn out. Ride and test the new setting. Eventually you should reach the point that you can't open the screws anymore due to the idle speed falling off and the idle stumbling. That is telling you you've gone too far and are past the "sweet spot". If you can't get the popping on decel to go away by the time you've reached this point on your adjustments, that would indicate that you probably need to increase the pilot jets a size.
5T, adopted your suggestion today. Went out with small screwdriver, stopped after a few miles, made a small adjustment both sides, rode on, repeated. The backfiring is definitely much reduced. Sometimes I can shut the throttle coming up to a bend and there's nothing, other times there's a muted pop or bang or 2.
Forgot to take me specs, so the mixture screws were just about a blur. Did small adjustments by 'feel'. When we were home, thought I'd better check what I'd done. Fortunately, both were set near as dammit the same, to about 2⅞. Feeling bullish, I've left them both at 3 turns out - see how that goes next time.
The bike is running real nice. Got the clutch so I'm happy with it. Getting used to the bike and didn't seem to suffer from jerkiness in the transmission.
Looking for other things to tinker with, decided it was time to have a look at the horn. Since I changed the horn for a
supposedly louder one, it has stopped working altogether. Not exactly a priority, apart from the fact that it's a legal requirement. A vehicle will fail the annual MoT unless it has
'audible warning of approach.' Fortunately, Miss November's mature years mean she is exempt from the indignity of having her bits inspected every year but it is still the legal responsibility of the owner to ensure that she would pass an MoT.
Took the l/h switches apart and dismantled the horn button to investigate. Noting wrong there. Seems the problem was poor earthing between the switch block and the handlebar. With the switches reassembled and the area that clamps the bar cleaned with emery, horn function is restored. It isn't very loud, somewhere between a polite stage whisper and a sheep farting but at least it works. Legal requirement met.
No photos of any of the work so here's a couple more garage shots: