I have done some fresh searches and it appears that the inrush current causes a B curve MCB to trip if the batteries are not connected. I hadn’t connected the batteries as I wanted to check the configuration and then connect the batteries (a bit of a catch 22).
The post in question is :-
Which MCB with MP2 5000 used for 3680 VA?)
Response 4 from Matthias Lange - M_Lange - Victron Expert
It’s not recommended to start the MultiPlus without a battery.
The high inrush current to charge the capacitors causing the breaker to trip.
If you connect the battery first that won’t happen and a smaller B breaker can be used.
I will connect one of the batteries and confirm that a type B is sufficient if the batteries are connected.