As it turned out, the diagnosis took less than 30 minutes, it was a broken power lead just inside the strain relief of the PSU. So no continuity one end to the other, but no visible damage.
A quick trim of the cable and a resolder job inside the PSU and hey presto, back to the land of the living!!
Considering the machine is connected via UHF to the TV, the picture is really rather good. I did have to remind myself that the sound came from the internal speaker though, and not the TV like the A8s and C64, and that I had to turn the volume down to 0!
I was worried that the keyboard membrane was dead, as some keys weren't working when I tested, but the ribbons were in pretty good condition, so I just cleaned with a bit of Electrolube, plugged them all back in and off we go.
I guess that means it's Mac Classic time next


