Well it wasn’t quite all. I’ve loaded up the car to take the wheels and frame off to see a Norton specialist tomorrow with a view to get the wheels overhauled with new rims.
I had a couple of hours to kill, ie stay out of the way of the wife, and so I had a pootle around in the garage. I decided to wash off the primary chaincases in the parts washer, and took a whizzy wheel and brass brush to remove hard stains. I found one horrid bodge where the drain plug had a stripped thread and in its place was a coach bolt and square nut!
On removing and binning said bolt I decided to have a look at the stripped thread hole and see what I could do with it. Unfortunately inside the case part of the casting was broken away and what remained wasnt very thick. I checked the diameter and it was close to being able to wind in a helicoil. I tapped the hole but the helicoil wouldn’t wind in correctly as there was insufficient depth of hole. So I simply tapped it out to m8 - yes, sacrilege on an old brit bike I know, but I don’t have UNC taps. I doubt I can trust the amount of threads left in the thin case, and unless I can get it welded I will probably JB Weld it inside and then drill and tap it further.