rcb trip find cause?

Random RCB trip on main split load circuit board.
no individual circuit appears to be responsible.
but will trip sometimes when cooker thermo static control comes on or off on oven. or if sometimes equipment is switched off around the house. The random trips could be fridges / freezers cutting in and out. No obvious faults with appliances. Plug sockets checked with plug in tester for correct wiring and earthing. suggestions anyone ?

gh
September 2009