here is watt (noice spelling, funny eh, no, ok then) I know.
RCB's have a rating for "leakage". all electrical units leak a bit of juice. flouresent lights leak, irons, kettles etc, if that goes over the limits, the rcb will trip. I am just in the process of building a couple of ponds and had an electrician in to do the wiring for the pumb and filters. he had a look at my current (no pun intended) setup and advised a seperate RCB for the exterior supply as my type of solutuon is prone to tripping RCBS and I would not want it going at random times.
Now, another story he told me was of a friend of his that had the RCB keep going at different times, but nealrly allways at the same time in the morning. it turned out that they had lots of electrical devices in the house (kids had major stuff in the bedrooms hi-fis, powerful computers plazma screens etc) and when the central heating kicked in in the morning, the extra leakage was enough to put the RCB over the top and trip. he proved it by changing the time his heating came on to a bit later and the same thing happened.
This is all probably of no use but I thought that i would share.
Edit: I just noticed you put MCB, dont know if RCB is the later version