Thanks to all for the excellent advice. I’ll need time to take in everything that has been said, but I thought my next and the easiest thing to do is move the throttle needle up a groove (as I see TT has just suggested).
Col, thanks for those two useful attachments and also to Orabanda for constructing his chart in the first place.
Since 2017, to give my GF every chance, I’ve been using BP Ultimate 97 octane and I add Redex Lead Replacement, Fuel System Cleaner and Sta-Bil 360 Marine Ethanol Treatment. As mentioned here before I just add this to a 20 ltr can, fill the can up at petrol station and top up my bike from it at home. The 13/32” BTDC figure is what I’ve read is correct for a 1960 - 62 Golden Flash in my books (but that was in the day). No idea what mine is at the mo. When I bought this GF early in 2017 the p.o. had only done 17 miles and that was to take it for its MOT. He had only owned it for a year after buying it from a fellow BSAOC friend who was a HGV mechanic and who rebuilt the engine. So engine wise it was already in good shape when I bought it. I’ve improved it in other ways, such as new 4 spring clutch, new sprockets, chains, wheel bearings, wiring, DVR2, remote oil filter, etc., but for the engine I’ve only ever needed to carry out normal engine related maintenance to keep it running smoothly, i.e. tappets, changing and adjusting points. plugs, ht leads, mag pick ups, etc., that’s why the actual engine internals and the timing, at least for now, remains unknown to me.
I've not had this problem before and I think it was a combination of the circumstances at the time that highlighted it for a 62 year old veteran (GF that is). So at least now I know and will try to do something about it.
Rog.