Well I finally put this one to bed today - sorry forgot to take photo's. I'd found that the zeroing drifted all over the place - I'd put it down to components marginally warming after switch on, but then I found that all I had to do to change the reading, was to breathe on the PCB that holds the load cell interface. Turns out that, as the gain of 75 of the op amp is set with a 10K and a 750K resistor, any moisture from my breath on the 750 k one significantly altered it's value
Thus the reading went out the window as I was pre-setting the trim pot on the board to it's nominal centre frequency.
The cure was simple - a couple of spray coats of 'conformal coating' (which is an acrylic varnish) on the boards after which you could practically dunk them it water and get no change
(I had to mask the trim pot and board connectors so not to gum them up)
Actually there is one more minor thing to do - the wall wart charger failed and I need to get one small enough to fit in the allocated space in the case - again ebay has provided one that's 'on the way'