I ended up buying a new LR rack for my new baby as I couldn't find the carrier bit (must have chucked it in the local skip thinking I would never have another L322 again 3 years ago when I bought the last Sport......... )
Anyway today I installed it and used my old drop-plate towbar with the old mount for the bike rack. All fitted together well and imagine my surprise with the old towbar in the new receiver, even unloaded not one rattle from the towbar even though I was using the old style pin and spring clip to keep the towbar in the receiver.

In the package with the new carrier is a complete receiver type mount with the threaded hole through the draw and a very small and flimsy spring pin to stop it undoing. One drawback with the new style mount is that you cannot use the bikerack AND tow at the same time whereas with my old system I can. 2018 Mini Countryman Cooper S E All4 PHEV in Melting Silver - it’s whisper quiet in EV and polluter modes