So the next job was to remove the electronics and power them up on the bench. The MI12 supplies 10 volts to the OMM so that's what I gave it. To my surprise although the power LED came on, the Yellow 'wake up' LED didn't. (it had in the machine) Investigating further showed me that the 1 second / 250 millisecond / 125 microsecond bursts were actually formed in the MI12 - the OMM just passes them on as IR - (this fact becomes relevant later)
Now what I wanted to do was prove that the IR receiver part of the OMM was (or was not) working. Using a remote control from an LED Floodlight, I clamped one button 'pressed' , arranged it to point at the OMM IR sensor, and poked about in the circuitry with my 'scope probe. Now much of the electronics near the IR sensor is covered by screening cans that are not easy to remove, and it's all a bit tiny being surface mount. However I did find responses to the IR signals that I was sending, so the IR diode must be working.
I then went to the other end of the electronics, where a DS8921M differential line driver talks to the MI12 - no output, and also no input. Now between where I found the response, and this chip, there is more hidden circuitry that I can't get at. But I don't think that there is anything sophisticated enough to be decoding signals, just buffering and level shifting. This is where the relevance of that wake up signal comes in. If the designer put the clever bit of the wake up signal back in the MI12 unit, he probably did the same for the touch received signal (maybe !!!!)
On the board are two miniature LM7805 5 volt regulators (that are working) and an ICL7660 switching voltage converter that is producing a -3.5 volt negative rail, but I have no idea if this the the 'right' voltage of course.
So in conclusion, it's inconclusive

The IR diode is working but I still don't know if the OMM or the LT02 is the actual fault !
Really I need to find another machine equipped with this system and wave my LT02 at it's OMM!!
In the end I put it all back together and took the dogs for a walk !