I’m just hoping there is some magic that will persuade MacOS to trust the embedded Ruby 18, but so far all it offers to do is move it to the bin (as the trash is now called in UK English after all these years).
OK, I fixed it by navigating to /Library/Application Support/TextMate/Ruby/1.8.7/bin in the Finder, clicking on the ruby executable, and then telling the system it was OK to open it, like you do with an unsigned app. A bit crummy, but it seems to have worked.
I expect disabling Gatekeeper would have worked too, but I don’t like to do that.
I don’t know why this happened – it may just be my setup and its history – but I’m posting this in case anyone else has the same problem.