Given I'd decided the Mockingbird was salvageable without too much pain it was time for the nasty monkey metal tremolo to go. I ordered a reasonable quality low profile one from CH guitars, it's no Schaller or Gotoh, but it is the sort of perfectly adequate licensed copy found on loads of midrange Korean guitars.
Once fitted and with the usual joy of floating/intonating a Floyd Rose from scratch it's obviously much better than the previous one. It was supplied with two new studs & bushes with a nice fine pitch thread on them. However I opted to leave the original BC Rich bushes and studs in place. If I ever refinish the guitar I may pull the bushes and fit these instead.
I also got a new 43mm nut at the same time, I'll fit this and set the height at some point soon.