Oh dear! I think I may have got you all searching for a solution for nothing. Sorry.
I just decided to look at the thing with the RF deck screwed in. It's not perfect but miles better than it was.
I did have a crock clip lead coupling the RF to the main but possibly not good enough.
See:
https://youtu.be/xXBL6R4ILLA
Peter
p.s. I don't believe it! It's failing again. Perhaps the mains was just in synch.