Well the Screwfix Masthead amplifier may have made a differance.
Not as much differance as unwrapping the christmas tree lights power cable from around the aerial cable behind the telly
All the problems went away, but as I'd brought it I then fitted the Amp anyway with the gain turned down low.
I did say it had only been bad recently, but had just not put two and two together, and didn't find it until I was poking around behind the telly to fit the power supply to the amp. I'd even unplugged and wiped all the connectors yesterday but just not untangled the mass of cables.
I can't even figure out how the cables got so wrapped around each other.
Stupid I know, but you live and learn.