I had a similar problem. I disconnected everything (so I thought) but the fault persisted. My fault was traced to my gas hob ignition system which was permanently wired via. a fused spur and so I overlooked it. Make sure you have definitely disconnected everything like your house alarm, bell transformer, central heating etc., things which are normally permanently connected to the mains via. fused spurs. Connect these items one at a time, test for a short period, disconnect and then try another item. Obviously, the one that trips your RCD is the faulty one.
You may have a faulty RCD, they are normally very reliable but have been known to fail.
Dave, June 2009