I finally removed my broken silencer - despite lots of heat and penetrating oil, I just couldn't budge it without making it even worse - if it's possible! So I decided not to try and repair it.
Matt at the DSS USA location said that they'd have a new stock of these after the new year. Then he got back to me and said they had a discounted one in stock with a couple of dents, which would be visible when mounted. Since I'm not doing a fancy restoration, and just want to get it running, I pulled the trigger (oh dear, that could have a number of meanings on this forum, couldn't it?) and bought it, along with a couple of other items on my list. The savings just about paid for the extra bits and the shipping...