I've deleted the media cache files. More than once now.
I couldn't get the cineform preset to give me anything but an audio file. I tested it with other clips to be sure I wasn't only testing it on what might be a corrupt file. Same problem. So went back to the old youtube preset which worked for that clip. I hoped that was the only problematic clip. But when I started using the next clip the same problem emerges - diagonal lines and nothing plays.
These clips play just fine from my computer where I dumped them from my camera. It's only in Premiere Pro that I have any problem.
The thing is, this video is precious. It is a hands-on record of a terrible multiple whale-stranding and rescue by Project Jonah and public volunteers on a very remote beach in Northland here in New Zealand. There are conservation organisations waiting for me to make it available, including the (Govt) Department of Conservation and Project Jonah. I really need to get this done.