Hi Martin,
I may have found a way out of this. I deleted the files in the MediaBay and MediaServices folders under Application Support/Steinberg, ran the Download Assistant and it replaced them. Cubase 13 now opens as it should. Presumably the files that were there previously were somehow corrupted.
AND
As a result of doing that, the VST Engine is now closing when I quit Dorico - that’s another problem solved.
Fingers crossed it’s all working now.