It seems that your main switch is an RCD. It is impossible for a manually operated double pole switch to trip no matter how much current is generated. It is not a fuse.
The type of fault you are experiencing is almost certain to be due to a leakage from a neutral to earth. It could be on any of the circuits because a main switch RCD is monitoring all of them. It is very difficult to diagnose a fault that occurs intermittently and randomly because there is no clear pattern and the interval inbetween the tripping is also random. There are many variables to consider so it would be a lengthy investigation. The first thing to elimimate would be the RCD itself. It may be over sensitive. If that checks out ok, then each circuit would need investigating. It would be a good idea to get an electrician on board to start checks. You don't want this to happen when the house is unoccupied when your away for holidays.

Would be best to call a Qualified Electrician, 100A BS60947-3 Main switches don't tent to trip unless the is a huge Short circuit somewhere. (You would know it would bang) More than likely what is tripping is your RCD. This could happen for a number of reasons Water ingress in external light fittings or sockets. Earth leakage exceeding what the RCD can tolerate. Would need a inspection to why the RCD is tripping. Cheers Callam.

As with any electrical fault, it would require a thorough investigation and fault find. There are many factors that can cause a fault, there may be faulty electrical appliances, faulty electrical accessories already installed, old accessories that are at the end of thier service life that may need replacing
It would be best to get a qualified Electrician round to investigate thoroughly.

Hi it would require testing. But as you’ve stated it happens at night when very little load is being used. It doesn’t sound like overload which is the common cause. What’s tripping the actual RCD on the board ? Also possible earth fault. Would need testing to see if it’s a certain circuit that’s been operated. EG lighting in bedroom or a socket.

At a guess without seeing the ‘main switch’ I would say that it is the main RCD that is tripping, not the main switch. The main switch is an isolator and therefore wouldn’t trip under fault conditions.
I also believe it to be an RCD as no individual circuit breakers are tripping. This indicates to me that it is a faulty appliance or an accessory which has had water ingress.
As the tripping is also random I would ask, are there any external security lights? Are they darkness and motion activated? Does the tripping happen more on wet days? My guess would be to look there first.