The ring allowed the house to be wired using less cable, one is permitted 106 meters of 2.5mm² in a ring however not sure with the rules about 1/3 in from end of beam if that is still true? I have wired a warehouse using all spurs from the ring, it had a 6mm² ring with a junction box above every socket with a 2.5mm² cables down to the socket, it was done this way to reduce the cable used in the ring and so keep the earth loop impedance within the required limits.
The installation needs to be designed, not simply thrown together, and step one before fitting a new socket is to work out if the ring can be extended. The Ze with a TN-C-S should be better than 0.35Ω but as to if you can rely on it being better is another thing, the street supply can change without notifying you so we work on 0.35Ω, so the line - neutral impedance at centre of the ring can drop to 0.94Ω and still be just within the volt drop limits.
The problem is measurement measure with any loop impedance tester three times and you will get three different readings, so if I read 1.35Ω the question is do I flag as a fault or not? Likely not as it could be my meter is slightly out. Even with a 0.90 reading am I sure I am in the centre of the ring. The only sure way would be to use a low ohm meter using at least 200 mA to measure with which would require the whole circuit to be cold. So switch off and then return in an hours time to measure.
If the loop showed as 0.85Ω OK I have some leeway and extending the ring is likely the best method. But you have only given half the story, without the test results no one can say if extending the ring is possible. In the main meters measure line - earth when set to impedance and line - neutral when set to prospective short circuit current, at least the better ones do. So looking for at least 244 amp. But some meters don't auto switch between line - earth and line - neutral I have been caught out myself and since the earth wire is thinner than the neutral one can believe the volt drop is exceeded even when really it is not. In theroy we should test both line - earth and line - neutral for the tripping current of the MCB we take the worse reading, and for the volt drop the line - neutral reading.
The testing is time consuming and you pay for that time, plus the cable costs and you pay for that cable, so often there is quite a jump in price between doing all the testing to extend a ring then use extra cable, and to simply fit a fused spur where if the ring was correct to start with then near impossible to get fail results when testing, the ELI jumps to 2.42Ω from 1.44Ω and similar with PSCC so that is the route many electricians go for.
When putting the ring in it is easy, 100 meter role and you know you must return to CU without starting a new role. But unless that electrician says on the paper work 85 meters of cable used, working it out latter is not so easy.