My thinking of what we have so far.
The catastrophic failure or the MOV (thanks) in the controller would point to a large over voltage situation.
It is most probably not environmental but a lost Neutral.
Without further information this could be anywhere internal or external.
So I was thinking to approach it on two fronts, to contact the DNO and report a possible Neutral fault whilst surveying the site, reported faults and drawings to see if a pattern can pinpoint the location.
So in a normal fault free situation the installation should look like.
If the fault is in the below position then I should only be hearing of power outages in flats 7, 8 & 9 but not over voltages or the MOV failures I'm seeing.
If the fault is in the below position then flats 4, 5 & 6 and flats 7, 8 & 9 will see voltage fluctuations between 0V and 400V depending on their respective loads, this situation could cause the MOV failures.
And finally, in the below image the fault would be evident across all phases and all flats with under and over voltages. Whilst there are sections of the internal installation that could contain the fault; in this scenario the fault could also be external.
Ii actuality I have already informed the client of the liklihood of a serious fault existing and that it may be internal or external and they will be picking it up from here.
I'm quite disappointed really as its quite interesting to see how far you can fault find on paper if you have enough information.