Hi Bob, these can be repaired by welding, or soldering a small strip on......fiddly, but do-able ....I would normally offer to help out mate, but work is really tricky at the moment regarding home jobs

To weld, it needs to be in the block, and tig is the process, use alloy steel mig wire with the copper rubbed off, and totally fill the vee.....then file a new one.

To solder...just shape a strip that is the width of the blade, and below the vee.....tin this with solder, and the blade, hold in place then a quick pass with a small jet, or touch with a soldering iron will have it done (wont look as neat as welding though).

Even a second hand BSA genuine one may not fit exactly, as these were fettled in.......I have found they often are stiffer / looser if changed between similar guns.....yours looks to be in very good order, so a welded repair would be the way to go IMHO.

Have an ask round, you never know who your mates know!