Ok, so did some digging. Powered off at the meter board, took the cover off the CU and tested the continuity of wires I marked as 1, 2 and 3 (see photo).
Turns out wires 1 and 2 had continuity, but 1 and 3 did not, neither did 2 and 3. So, as was suggested above, it seems that one side of...