Hi Roger,
Yes, the sprocket is bolted on as you say, and the rim is spoked to the hub both sides and not to the drum. I think later Dragonfly models had a British hub which had a different arrangement with spokes to drum, but not on a mark or a comp.
Good luck finding a hub if you haven't got one, (I may have a spare), and even better luck finding a drum! I gave up looking and made one recently from a Triumph part.
Cheers, Neil