Following lots of reading from the many amazing websites (*, thank you) and poking around inside the machine, reseating chips & cables & checking the IC, everything was pointing to faulty ram. So I ordered 4 ram chips from ebay (just a few quid) and they arrived today.

Thankfully the ram chips had sockets so they could be replaced without having to switch on my soldering iron, just pull out and carefully replace.
(The ram already in the machine was of all different makes, so I am not sure if they were original and if the ram sockets had been installed later on. Mega result, as saved me a lot of time.)

I took the machine apart, and began replacing the ram chips one by one.  Cycling on/off between each ram chip replacement. After ram chip number 7 I found the culprit ..YYYYYEESSSSS
Had it running fine for 30 mins, no problems. Plugged in a Kempston Joystick and loaded  ‘Turrican‘ and ‘Chase HQ‘. Both played great.
Computer connected via RF so picture quality was poor, so I need to invest in a better tv connection method.

Breadbin back from the gaming graveyard

*source site used to help me diagnose the fault, Ray Carlsen’s Home Page