An update...thanks for your ideas - I opted for getting the leg welded up, but as my welding skills and experience are non-existent I popped over to a local welding business down here in Kent and they filled the area in with alloy weld (along with a few other nicks and chips) for the princely sum of £40. Bargain.
I attacked the resulting metal blobs with the mighty dremel (other rotary tools are available) and did what I think was a passable job. I was going to take a photo to impress you all with, but decided to put the leg back together again to make sure that everything fitted as it should - and realised once I'd completed the work that it DID all fit, and I wasn't going to pull it apart again.
Anyway - the proof of the pudding is in the riding, so once I get my newly-chromed exhaust header back from the chromers and put the bike back together I'll take it for a test run. If it leaks then I guess I didn't do the passable job I thought I had...and will need to break out some gasket sealant or something similar to seal it all up.