Yes, Martin, Mark and Bill that was it. Opened the case and the keyboard pcb end of the cable was off by one pin (unconnected pin was to the left, as viewed looking from end of the MTX with the external edge connector). I didn't notice, but I must have done this when I reassembled after the (very necessary) deep clean of the keyboard.
Point taken Mark, about double-checking before hitting the power button. And yes, the reset buttons did work when pressed even with the cable off by 1 pin.
Thanks again folks. I'm going to teach the kids a little Memotech basic tonight and next, I'll need to think about getting a CFX.
About original Memotech hardware.