Mains recurrently tripping

Hi,

I wonder if anyone can help!

Ever since we had a kitchen diner extension 3 years ago our mains electricity has randomly tripped - sometimes 3 times a day, sometimes not for 2 months. We haven’t found any rhyme or reason to it. Last time we went away on holiday we turned everything off, unplugged everything except white goods and came home to find our freezer defrosted and power tripped. 
It is the red main switch in the fuse box that trips. 
The extension was done by a well trusted workman and electrics by a registered electrician. A new fuse board circuit covers most of the new extension electrics. 
A new boiler and new outside lights were put in at the same time as the new extension. The boiler service person couldn’t find any reason that the boiler would do it. We paid for an electrician to come and do an electrical safety certificate and check everything out and he couldn’t find any reason for it.
We’d love some suggestions for anything else we should try to track down the cause!! 
Has anyone got any ideas? 
thanks

Zo

Has this big red main switch got a tiny little button that has a ‘T’ on it? 

Random tripping can be the worse thing to track down - however you mentioned it tripped when you were away and unplugged most appliances.

I tend to suggest customers keep a record but in your case it points at a wiring issue or the fridge freezer

Does your "extra" fuseboard have an upfront RCD (main switch)?

I'm assuming it is an RCD that trips, (?), then:-

Outside lights are always number 1 suspect to me.  Look for moisture having leaked into the fitting or associated junction box.

The auto defrost control/heater on your fridge freezer has been named as suspect a few times on this forum. 

Do you have an immersion heater?