Thanks Roger,
I have attempted to remove various cards in order to isolate the faulty section - I will try this again and take some notes of my findings - in all permutations, there is not ever a situation where the screen displays sensible text, just differing quantities of rubbish.
I am currently rebuilding the power supply with new electrolytics - reasoning that a noisy SMPSU will not help the processors lead a happy life.
I am in touch with the owner of Kelvedon Hatch Nuclear Bunker, who has at least three of these machines, although I am unsure if any of them work - if one of them is functional, I will see if I can substitute my boards to try to narrow down the problem one.
It is somewhat frustrating - I bought the machine from it's original owner, who used the machine right until the Telex network closed in the UK.
I have to say the thought of drawing out the circuit does not fill me with glee.
I am sure having a logic analyser would be helpful, but only if i knew what I was doing with one
Cheers
Sean