You are right! I should have brought at least one counter
I usually do. However, I'm working away and staying on site so I wouldn't really be going anywhere, and where I am is pretty low background as
there ain't much uranium underground around these parts.
I'm back home for the weekend so if I get chance (emphasis on
if) I'll see if I can workout a better pinout of the ribbon cable as I am interested in getting to the bottom of why yours isn't working. And, from what I remember, the ~4V feed to the front panel PCB is also for the LCD too, and maybe the bleeper (button presses/alarm) so it seems strange that everything else works on yours without that ~4V feed.
Edit: I just checked UK radon maps and I'm currently in a >30% radon area, so there Uranium underground. Lots of it according to the map. So I will bring a couple of counters and my Radon meter next week when I return to site.