I denitely think its the wiring. Cable is waterproof son unless damaged, is unaffected by drips (unless the water gets into an outlet or connection box).
All your appliances, and the boiler, are protected by a 13amp fuse in the plug or the connectyion unit, so the individual fuse would blow before the 16amp breaker.
If you've had mice eating the cable, that could do it, or if you'#ve put a drill or a nail through a cable, e.g. buried in the plaster.
A qualified electrician wil trace the fault by measuring the resistance in the circuit, and at points along it (he can even calculate the distance from the test meter to the fault by the resistance). The electricity Supplier wil provide someone if you can't find a reliable and recommended one.
MCBs have two ways of tripping - fast, with a heavy overload, and slow, with a slight overload. They don't store up a charge.
How old is your house, and your wiring?