Just an update on my experience. After becoming horrified at the cost of a replacement mirror, even second hand, I set about removing the mirror and stripping it down to see if I could effect a repair.
Short answer is, I have. Sadly, I did not take pics of the steps, which may have helped others who are happy to have a go. If you do end up being one of those people, contact me and I'll do my best to assist if I can.
How much did it cost me? Nothing. I found a metal tube with a flange that I found in the garage (pic of similar below), got out the angle grinder and 'expertly' milled it to an interference fit and pressed it into place using a nut, bolt and a couple of washers for a test fit, then couldn't get it apart!
It survived speed limit plus a percentage testing over a week or so.
Tube with flange looked like this:
Hardest part was getting the grease off the kitchen worktop. Wife moaned but I kept reminding her why.