FIXED!!!!
The problem was indeed a faulty U850B (4053B). I removed the A1 (an interesting challenge!!!), de-soldered the chip and inserted a socket and removed a 4053 from a parts mule and hey presto, TWO Cursors!!! Just as well I have a nice de-soldering station.
SOMEWHERE in my attic (I'm sure) I had/have tons of 4000 series but I have no idea where they are so I was forced to re-use a device from another board; not ideal but hey it worked (silly prices on eBay).
Just a minor issue but still haven't got a true zero brightness when the Readout Intensity is set to mid-way; As far as I understand the display should blank 100% at mid-point? Haven't noticed any adjustments in the set-up for such a condition. Not particularly important but would be nice to get it right.
