Yep. I think it's more bizarre because I've triple checked the connections, and can't see anything wrong, with both the original socket and my new one.
That is weird. Essentially, to complete the ring, the incoming wires just need to be in contact with the outgoing wires. You didn't tug really hard on either cable, did you?
beseku
rodabod
ChainBreaker
Surely the ground floor and loft work on separate rings.
Pop that single socket off again (with the mains isolated on the MCB) and check that the incoming and outgoing cables are securely screwed in.