I was attempting to to fathom the inner workings of our electrics today, and got some things sorted. So at least the whole house RCD is gone and we should be able to endure a horse farting without it tripping!
I found one of the ring circuits with a third cable joined to it at the CU. So having tested which were the ends of the ring, I thought it would be easy enough to see what this extra one was by the expedient of leaving it disconnected and seeing what no longer worked. As far as I can tell alas - nothing!
All I can tell so far is its a 2.5mm^2 T&E that romps off downwards. The L to N resistance is >20M Ohms (on a DMM). I stuck a tone generator on it and could follow it only to arms length down form the CU cupboard, but could not pickup the tone anywhere else. So unless there is a socket hiding somewhere and it was just a spur taken from the circuit origin I am not sure where else to look. Suggestions?