Sorry everyone I hadn't kept an eye on this post. After waiting for two electricians to contact me, I managed to discover the cause myself. I had originally unplugged every appliance I could think of and the circuit breaker still tripped (didn't know there was more than one
Eventually the penny dropped and I remembered that I had an extension lead going from an outdoor socket to a freezer in the shed. When I checked the lead it had been almost eaten through by a mouse or rat. After I disconnected this lead everything was fine. I was so pleased with myself it was as though I had just invented gene therapy!!!
Anyway I was hoping Sanchez would have got in touch as I would like the name of someone qualified to keep handy in case of emergency. His PM cache was full so I emailed him but maybe he is away on holiday.