Success! Have had the desk returned, repaired by the engineer that "badgeronkeys" used in Devizes, UK.
The repair notes state: Voltage Rails - 16V overloaded. Fault traced to Control Panel (13-24) - 0.1uF x 1 capacitor short circuit. Replaced all x 34.
Preventative action - replaced all 0.1uF on Control Panel (1-12), replaced x 52.
Brain PCB - ADC regulator circuit: replaced 6 capacitors (0.1uF x 3, 10uF x 3).
All header connections treated with contact cleaner and lubricant.
PSU in PC checked, ok. Cable and BFC to mixer ok.
So quite a job, and a really reasonable cost for 10 hours work. Delighted to have it back and with no issues at all. If anything the operational speed seems nippier, even though the software was not re-installed.
Thanks again to "badgeronkeys" for the engineer recommendation, "hyaena" "anyhorizon" and Bruce Graham for encouragement.
Here's to a few more years of service from a 16 year old desk!
Guy