Have the problems started since the builder came to visit? Have they done any electrical work in your CU?I have also had my light fitting down recently too (as a builder knocked it with one of his tools and it blew the dimmer switch somehow). Since then I have removed the dimmers and replaced them with standard light switches - which going back to the start of this thread, are the bits I have disturbed.
Does the RCD protect the socket circuits only or lighting too? Is it a dual RCD board or a split load board?
If you could post a pic of your CU and the incoming supply area it may help.
That's the weird thing - since he cleaned up and packed up, we've had the kettle on the go - as well as this mammoth industrial carpet cleaner. I can describe the boards for you... Basically it starts off with a 100A cct bkr going into the meter, then out to the MCB board with the old style fuse wire cct bkrs, 2 white (5A), 2 blue and 2 red ring main - with an ON/OFF switch on that ...then it goes into the RCD unit (not sure if 30 or 100).
I think maybe I should just kill the power and take each socket off the wall in turn to see if there's any neutral cables that have been crushed back against the metal case. After that, I'll check the outside light for signs of moisture ingress ...and then if still no joy, do what ericmark suggests and go with the electrician idea.