The RCD on our house pops at seemingly random times. 

Just came back from holiday to find our power off due to a RCD trip-- luckily it was just the day before as the freezer was still cold. 

But, if we go on a longer holiday it could be a disaster, with our freezer and fridge going off. 

Is it possible to even diagnose this for a reasonable price?

We did have an earlier issue where the ice defrost heat element in our fridge was causing constant RCD tripping --- but that was fixed early 2015 . 

This seems completely random and reasonably infrequent (5-10 times a year maybe).