Yes, the vcr is old and probably about to be replaced.
I think it may be near the root of the problem.
I changed the rf output on the stb from 54 to 32 via a few others that didn't help.
It all seems to work fine now - but only when the scart between the vcr and the stb is not connected.
I may bring forward replacing the vcr (probably over 12yrs old now

) and pop in to Richer Sounds tomorrow.
Thanks very much for everybodys help. please keep suggestions coming, and I'll update as I get further in/out of this mess.