HOWEVER once clips are dragged to the timeline the clips WILL play without any problems, both audio and video - there is still no thumbnail reference image generated however. I'd be grateful if anyone could suggest a cure for this problem. I will add that other 4K clips shot in .mp4 format are not problematic in this respect.
PS a friend has PD18 and he can see thumbnails on the .mov clips, both in the media room and the timeline and the clips play fine in both areas.