Hi Steve,
Luckily, I had the correct new rear vented disc, identical in every way to the original disc.
I tried a dry run on the bench and found that the disc was too thick for the groove in the new carrier and the groove in the new calliper.
I then realized that the callipers that I had were for a solid, thinner disc so I didn't even bother trying to fit them.
I now need to open the EPB to fit the new rear vented disk, no point in waisting it.
When I have eventually got my sand blaster up and running I will clean and paint my existing rear callipers.