I have a Wylex consumer unit fitted at home with about 8 seperate MCB's for the different circuits. These are the clip in replacements for old cartridge fuse holders.
Over the last few nights the main RCD switch has been tripping. This is happening in the early hours of the morning when all of the family are in bed, and switches all of the power off lights, clocks etc etc. There is nothing that switches on in the night. Our CH boiler is a condensing combi boiler which is obviously on 24 hours.
The problem has only started over the last few nights and not happened at all in the day. I cannot think of any new appliance which could have caused this.
If you have outside light(s) it could be that condensation is forming around the cables and causing tripping. This would only happen at night when the light comes on and warms things up, there will always be a delay, it is not an instant process.