Removed and isolated wire #204 & 207 from J5-1 and J5-2 on the controller board, everything fires up perfectly, and no tripping.
I still have the coolant pump working and delivering pressure just fine, but only via the manual switch on the cabinet, rather than through software or machine control.
Been test running the machine thorough some warmup programs for a couple of hours and all seems fine.

I'm going to check the pump out separately over the next few days as I need to do a coolant change. But I'm sure it's ok, as it's still going through the original supply circuit, and any earth faults or shorts would trip the mcb immediately.
So it looks like the fault is either in the connections to the board or to/in the K5 relay