Just been checking some wiring done in the era of a previous owner of the property. A ring main, where each double socket had one of the cable earths connected to the back box, the other to the earth terminal on the socket, but no box to socket wire, so ring earth continuity relied on all the back box screws. (now re-wired). Then two outside MK waterproof sockets, each a radial off the consumer unit rcd protected side, with the earth connected to the terminal on the plastic housing but not to the socket, so no earth at all!
Just as well I checked !
When putting in ring mains, I've always individually sleeved the cable earths, tight twisted them at the bare end then put them in the socket connector with the backbox earth lead. This way even if the earth terminal screw worked loose ring earth continuity would be preserved, but I understand this is frowned on now as it makes breaking the loop for testing a bit harder.
AWEM