Thanks to a process of elimination, the display problem is due to what appear to be an internal short on one of the 7 digit alphanumeric displays. One display works perfectly until I plug the faulty one in, and the when the bottom RHS segment on the far left and centre digit switches on - it also switches on the lower horizontal segment. This also happens one the "good" display.

Bloody williams displays !!

So I will need a replacement display - Part number 5769-10874-00 (7 Digit Alpha display). I will post a request for one on the "parts wanted" section.