Thank you for your responses, everyone. The MCB tripped once, but the thing that trips every time is the RCD/RCBO - I don'tknow the difference. The one in place when I started was a Contactum one marked 7132B (B32). I replaced it with a new one (just in case) which has a slightly wider switch but the same serial number. The MCB tripped while the circuit was off at the RCD/RCBO while I was chipping out a metal box on which I saw signs of rust. I wondered (since it was on an outside wall) if the recent heavy rain had penetrated, but although the cement and plaster around it had deteriorated, it didn't seem damp now. I have left the space open for 2 days, and warmed the cable ends with a hair dryer, but when I restore the power, it still cuts out. Usually it cuts out straight away, but sometimes it stays for 20 seconds or so, and then cuts out. I don't have any special test equipment, which is why I thought of splitting the ring at every socket (heaven knows where the centre is), detaching one end of the circuit from the consumer unit and then powering the other one up, and restoring one socket connection at a time until the RCD tripped again. Then I could do the same to the other half of the circuit till it tripped. Presumably what was left between (what tripped the RCD both times) would be the faulty part. But this will be a long process ! I suppose it will be satisfying if it works out (possibly even entertaining!!), after which I can tackle the laminate floor (or install surface trunking, ugh! everywhere). An added irritation, but not perhaps relevant, is that the walls are very hard and the cables have been plastered in with no conduit or trunking, and the tails in the boxes were left so short that the installer had often to crimp bits on the ends in order to fit sockets and switches. And it pased inspection ...