It's a relief that you guys are almost quoting the conversation I had with the owner!
I'd eventually written the 6mm off as (probably) an old shower circuit. I left it disconnected at that point.
I later did a walk around with him to find out what didn't work as I'd also got two 2.5 cables disconnected that had been both in a B32 but I had no end-to end readings.
We found three double sockets in lounge not working.
Then a bit later we found a macerator en-suite toilet (which was directly above lounge) not working.
I temporarily connected each half of the non-ring to a B16 and found one cable got one corner of the lounge working, the other cable got the other corner working. Neither cable got the macerator working...
It still didn't dawn on me until I took the FCU apart and it was the 6mm feed, almost certainly the old shower circuit from the next room repurposed. The guy (who was Dutch) found this really funny and said does that mean I can do a 32 amp sh*t then?