Thanks for the link. After contacting futurlec to check if they had it in stock, I ordered the last ADC they had.
Having removed the ADC I checked the signal on pin 5 of the 8051 replacement but rit didn't show the ~80Hz signal., unfortunately.
I also bought a logic analyzer
https://www.embeddedartists.com/products/labtool/ but installing the drivers is a royal pain and I've not managed to get it to work. Contacted their product support and am awaiting their reply.
I hope to identify with it why the uC keeps receiving the reset signal.
Till I get it working progress has halted, unfortunately.