Hi Pat..There are UK manufactured drums available. Velocette, (no connection with the bike),in Coventry make them.
They are a trade only outlet so you will need to ring around and find a BSA dealer who stocks them by specifying UK manufacture. I would try some of the better known ones like C&D Autos, Lightning Spares and some of the Gold Star suppliers who are more inclined to go for the better quality parts. You could also try a friend of mine who doesn't sell Indian stuff and does deal with Velocette..Monty's Classic Motorcycles..01822 617010....Ian
email (option): ian@wright52.plus.com
Hi Lee..Pats bike is a plunger model with the later wheels and drum. Was your drum from Russels a NOS item or a UK made replica..and how much was it?. I had an Indian sprocket that came with a job lot of spares I bought..In my eagerness to fit a 'new' sprocket and drum I didn't spot the fact it was a replica and fitted it to my own bike. Bad move..within a month 6 or seven teeth had snapped off at the root. Further investigation with a lump hammer revealed it was made from cast iron, not steel 
.It's probably back in India by now being made into an M20 crankpin..
...Ian
email (option): ian@wright52.plus.com
.It's probably back in India by now being made into an M20 crankpin.. ...Ian It's a UK made replica that the guy from Russells had commissioned. He said that his Indian replica was shocking and got a firm in this country to make a batch. I've yet to ride the bike but the fit was very snug with all holes being where they should. I could be wrong, but I think it was somehwhere in the region of £65-70.
email (option): lee@twowheelstested.co.uk
Thanks Lee...Ian
email (option): ian@wright52.plus.com