10,000 thank you's to those on here (especially Amauget) who responded to my constant questions as I tried to get this machine to work. The final exercise was to extract all of the memory chips, replace row C with new chips from an Atari 16k card from ebay (sorry to the Atari), flicked the switch and it beeped for the first time in many years. No more hot ram and screen full of lines of characters, it was back to life (other than some lazy keys which seem to be resurrecting).
Just need to figure out which memory chips were bad and smash them with something heavy and blunt.