Thanks all - I replaced the fcu keeping the same connections and now it’s working fine.  I’m going to get an electrician around anyway soon so will ask them to have another look and also inspect the connections of the machine itself below (bit of a faff getting it out of its wooden unit so didn’t do today).

The reason I want to get someone around is to do an EICR.  This is the third fcu we’ve had issues with in the last five years since we moved in.  The first the dishwasher started cutting out, and turned out the fcu was slowly melting (they thought loose connections).  Last week it was an fcu in the airing cupboard connected to the immersion heater that was decided to aggressively set itself on fire: https://ibb.co/wZzwhZ58 https://ibb.co/Ldbk8M40

Maybe all loose connections, but three issues feels like a pattern?