Hi All,
Before starting to tackle this myself I had one more search, and found this place up in Scotland which looked as if they might be able to do the job, so e-mailed them and asked. Got a reply this morning and they've quoted me £30 - £50 plus postage (c $45 - $75 + postage).
This might seem expensive as I only paid £28 for the mirror in the first place. But I plan to send it to them with instructions to do the work so long as it falls somewhere near the lower end of the estimate, which it should (I hope) or return it and it will only cost me the postage .. Also I hope that if they're actually holding onto it they may think that they might as well do it. So I could end up with a nice Edwardian, Chester hallmarked mirror for under $100... which doesn't sound so bad.