Main fuse blows, individual fuses do not
In places where people say "knicked by the bohrdrill", it's common practice to use whole-house ground fault detectors (they say residual current devices) in the main panel (they say fuse box). 

The two 40A boards is not weird in 230V land.  They bring 3-phase ~400V power down the block, 230V to neutral.  Having two boards means he is getting 2 phases.  

In American style per-circuit GFCIs, it matters a lot when you cross up neutrals between circuits.  A whole-house GFCI/RCD won't care about neutral mixing among circuits out of that panel. ** But two whole-house GFCIs will care a great deal if circuits from the two panels have their neutrals crossed.  That is also super bad because of the 400V potential between the two hots. 

I gather this recent work has to do with adding the second panel and moving any circuits over to it?   That will expose any crossed neutral problems.   

Of course, crossing any hot to any ground, or any neutral to any ground, will also trip RCDs.  

Sometimes the problem is only provoked when current flows, e.g. Neutral-Ground crosses.   In that case some current is going down the wrong wire.  With no load, half of 0 is 0 and that won't trip an RCD.  



** Note that neutral mixing is still bad, since neutrals don't have fuses and rely on the overcurrent protection from their partner hot.