I suppose we should write a standard reply for this problem, it happens so often,
@pete01 is correct for step one. Last thing any landlord wants is to call out an emergency electrician, that's expensive, so better if swapped to RCBO's, that way only one circuit is lost at a time, so fast response is not so important.
But his reference to neutral earth faults, is a good one, unplug rather than just switch off where you can, this ensures both lives (line and neutral) are disconnected. With a FCU switch off is often better than removing the fuse.
In your case it seems you have off-peak power, I note the "contactor" written under four of the MCB's one says "Time Clock" and we can see one to left, it is important this is set to correct time, but also it means storage heaters and water heater are only powered as set times, they all use mineral insulated elements, the mineral is hydrophilic, i.e. it attracts water, so the slightest damage to the seals, and it can start tripping an RCD. The same elements also used in cookers, freezers, washing machines, but in your case, if on resetting it does not trip, then may be worth temporary just long enough to test, turning on the time clock. Make sure set back to correct time after.
As an electrician, I have three tools to help me.
these are not cheap, so not the tools a landlord would really want to buy, there is a fourth machine, we often call it a PAT tester, the problem a landlord has, is he can test his own equipment, but has no idea what the tenant may plug in. The first machine shown is often not used, but on the main incoming tails as shown, it shows the total background leakage, and allows one to test all together, and can show where the faults are a combination of items how they mount up. I find it handy to see if we have something getting near to tripping the system. My tester cost me £35, but many only measure in 10 mA increments, so need to be careful it will measure in 1 mA increments.
But the real question is when is it not worth DIY, at what point does one say, call the electrician? The other is, if the fault is something the tenant owns, how would they feel paying the bill? Never lived in rented property, so don't know.
