John,

Thanks for the reply.

Some CUs come with a link wire between the neutral bus bars. You need to
remove that when using it as a split load unit.

I removed this prior to installation.

A faulty RCD would be aother possibility. Difficult to test reliably
without an RCD tester though. You could swap it out for another to be sure.

I don't have a spare but, I will ask someone who has a tester to check
it for me.

Or it may even be telling you that you have genuine circuit faults! A
quick and simple test would be to check for any neutral to earth shorts
at the CU end of the circuit wires, before you connect them back. You
can get useful information with an ordinary multimeter.

I will check this when I get chance, is this simply a matter of testing
for continuity between the neutral and earths for each circuit at the
CU?

When do you get the trip? If for example you connected a socket circuit
and made sure there was nothing plugged into it, and any equipment
directly connected was disconnected at its FCU, does it still trip with
no load? If not does it then trip with a small load applied (like a
table lamp for example)?

I will check this when I get chance, thanks for the assistance.