Ok, here's an update....
Put my ailing Miggy on a Scart lead and the picture is absolutely A1
This is telling me that Denise is alive and well and leads me down my original thought of something after the decoder chip having gone South.
At least I am making progress at last, I thought I might have to borrow an oscilloscope to track back the ripple on the signal; I am going to change C236 and C237 as I originally planned and report back.
Merlin