I have a newly installed Mem2000 split load consumer unit. When first
installed the RCCB tripped when the test button was pushed. I have
added a new socket and now the test button does not trip the RCCB.

Hi James,

Interestingly, I have got a MEM one on my (long) list of repairs to
do.
Its at a relatives house, at it too does not trip when the button is
pressed, nor does my RCD tester trip it.

I believe all MEM RCCBs (RCDs & RCBOs) are electronic-based devices -
they almost certainly will be if they have the extra functional earth
wire.
These can be damaged by an insulation test (so called Megger test) on
the wiring if they are left in circuit. Was an insulation test done
when the CU was fitted?
John