Thanks for the info guys, but I already had the Thundercat complete rear wheel assembly (except that I seem to have mislaid a spacer).
If all goes to plan, the only two bits of work to do are taking some material of a spacer and the caliper carrier, then that seems like a real result, with minimum fuss and work.
I'll let you all know how it goes.
I've also got a complete EXUP RU front end (with Ducati discs, Fazer master cylinder & "blue spot" calipers) to go in, but am waiting for the bottom triple clamp to come back - prior owner had fitted the forks & clamps into a Kawasaki and had swapped the stem.
Mind you, if anyone has a spare triple clamp with standard stem, that would be a quicker/easier solution.