Alec_t - You may have a point there - I'll try increasing the supply voltage (currently 5v) to allow a higher Vmax input at full scale, though my test is only using 3 x 3914's over 3.5v so should be 0.111v, but as you say, a bit near the knuckles maybe.
JimB - A shame, but thank you.
Chemelec - Thank you also, but my ground points are as physically close to each other as is possible! Still, could be an issue, which I'll investigate.
In general, my firewall doesn't like any of those links, and (this being Windoze) I'll avoid them for now and persevere (good job there's no deadline for this project!).
The odd thing (to me anyway) is that it works perfectly in bar mode, so if all else fails I'll decode that (40 x ExNor gates).
Thanks once again to everyone.
I just have to add this - we went to BC in 2012 and it is stunningly, stunningly beautiful. Sorry for deviating from electronics!