Many thanks for all the comments and suggestions. I have been contacted by RGS Bill in Seattle with a wonderfully detailed guide to running in, and have as result ordered five litres of non-detergent SAE 30 from the Vintage Bearings Company. Less than £20 delivered seemed rather good price. I won't be trying multigrade, and I'm pretty sure I don't have an auxiliary filter unless it is really well hidden. Damn. Will miss the Ashford autojumble because I'm working but strangely we had a VMCC motorcycle jumble last Saturday at Dunkirk village hall, which is about three minutes away as the BSA flies. No prop stands, but someone had just sold a genuine Goldie frame and an A10 Super Rocket engine to an enthusiastic youngster. I have ordered the prop stand from Draganfly, which seems a rather useful and friendly company. And my bike still doesn't leak oil despite being full to the level.
Colin Seeley worked at Schwiesos? Blimey. Didn't know that. I remember the closing down sale well, with a new Matchless Monarch at a daft price that I still couldn't afford as a 16 year old, and even if I could, I was limited to a 250 anyway. My first bike was an Enfield Crusader Sports which had been flogged by a gentleman named John Surtees up near Eltham or Sidcup or thereabouts. I wonder what happened to him...