When I found the problem I thought straight away - I bet Ian would have told me to check that track!
This was a unit from a well known auction site. It looks like a previous user had replaced the battery (it was an Ni-MH) but looks like they hadn't realised the old one had damaged the traces under it. This had me fooled for a while as the battery looked new (no fur jacket!) and the battery voltage was within range (3.7V) for a powered on and powered off unit. Keith got me to check the SRAM supply at its pins which was way out at a scary 10V - which would relate to the charging supply from the +15V rail via the 2k2/1N4148 - but without the battery acting as the voltage clamp. I eventually removed the battery as I wanted to check all continuities with it out of circuit and voila instantly saw the track. Buzzed it all out and realised this was the problem. I cleaned it all up thoroughly then used some mod wire to reinstate the connection. The other traces looked ok. After that we were up and running.
The only annoyance is I should have got the thing cheaper as it was sold as working - well it did power up - but with the 012 error code on the display!
Still all's well that ends well!