Time to find out what these MCBs control and investigate, I'll bet the damp has got in or the local wildlife has had a nibble.
If not already done, pop off all your outlets and light switches to get any damp or creatures out. Madam's hair-dryer is a useful tool!
Since you can get two MCBs active, check what they control (and record) so you at least have an idea what not to look at.