The testing of electrical work is a problem with DIY, most DIY people simply don't have the equipment even if they have the skill. Theory we should simply say don't DIY, in practice people will DIY so giving some guidance seems the right thing even if not 100% correct.
So you need to be able to "bell out" cables, we call it bell out because in the old days people actually used a battery operated bell. So as long as you have RCD protection, if not then you really do need an electrician, then with the power off, (Main isolator or RCD as a MCB does not turn off neutral) then you take each pair of wires part them and test with multi-meter to see if connected, then also test between the pairs to see not connected, this is why main isolator if you only switch off MCB it will show neutral connected to earth.
Once you have verified it is a ring the next is to decide how to extend, using a grid plate and a pair of sockets you can turn a double socket into two singles without swapping back box, then you can extend the ring from that point, or you can fit a single socket a switch and a fuse in the plate so you have one switched socket and a fused spur, in theory you should test the loop impedance, the EZ150 costs under £50 and will test the loop impedance however the first light is 1.5Ω the pass for ring is 1.38Ω so to get a meter able to give you a figure rather than pass/fail the price jumps, to a point where likely cheaper to get an electrician to do the job.
Even the
firms can get the testing wrong the link shows how it was a combination of plumber, plaster and electrician which caused the accident, and the guy blamed did not even go to the site. The person who did the testing got it wrong and the foreman was blamed for sending a semi-skilled man to test it. The main point however was the electrician doing the wiring did not really make a mistake, it was the plasterer who connected the line to metal work, so it does not matter how good you are at installing with so much going on it really need testing.
This happened before RCD protection today one would hope the RCD would trip, but again the tester for the RCD is over £100, you could do the work then get an electrical installation condition report done, however not sure if that would then save you any money?
So the point is however you do it there is a risk, and you need to assess the risk, and decide if it's worth it.