Ok I can confirm that this is now fixed and working.
Fault 1: Not powering up, displaying random digits etc
Fix: EPROM was faulty, replaced and copied software over from working unit.
Fault 2: Not booting fully, gets as far as displaying 1992 and then freezes
Fix: IC28 was faulty, IRQ line stuck low causing microprocessor system to be in a constant state of interrupt.
Hopefully this helps someone else fix one in future.