I was very patient with mine. Applied jollop then heat and as carefully as I could worked a tyre lever around the edges until it came off. I then had to do some tin bashing to straighten it out. You cannot get to the bit that's on the hub so there is every opportunity to bend the outer part. I imagine that back in the day the old one was scrapped and a new one bunged on. When I refitted mine I eased it with emery cloth and greased it.
I think this is the item. From the BSA parts catalogue: