...there were two oil tanks that came with the bike, one has a hole where the threaded fixing boss for the spacer bolt connecting to the battery carrier has broken out. The other one has had the same boss and threaded hole soldered up, presumably because it had also been damaged and was leaking.
As the tanks are fabricated using soft soldering, after removing the paint, I decided to desolder (using MAP gas and an airline), clean and re-solder the best of the two tanks
I took the opportunity to knock out some small dents in the tank while it was apart.
When reassembling I made a larger doubler plate to reinforce the threaded boss that was leaking.
Upon inspection of the connection fittings - the filler neck, oil outlet connection and drain plug fitting all looked sound, but the oil inlet connection had a crack in the solder on the inside. So this was removed and cleaned also and refitted with a reinforcement collar.
After tinning, I re-soldered using 40/60 tinmans stick solder and a butane torch.
Seems to have sealed up well but I will leave it sitting for a few days full of white spirit as a leak test, I'll then low pressure air test it and inspect using snoop liquid.