Well, I am still alive.
So I managed to find my clamp meter and it is showing 0.15A to 0.5A on the 20A setting depending on if I clamp near the RCBO or the main tails where the 4mm cable is accessible.
Don't think one can trust it... Ideally need something that measures in the 2A range. Will be worth it in my electronics lab so I will grab one tomorrow.
Got my hands on the multifunction tester. A pretty good bit of kit to have a play with. Found a fault on an old extension cord (not been used in years) so that has gone in the bin so already paying itself off.
I did the RCD tests on both AC/AC+DC settings. The 30mA RCBO for the sockets was fine as was tripping within spec.
The 100mA RCCB tripped at a max of 105mA on AC and at 450mA on AC+DC. Not 100% sure about those results.
I did an IR test on the board between N+E and L+E * 3. All fine.
Having been messing around with all the plugs and sockets (mostly extension cables in the office area), I have reduced the apparently N to E leakage down from the average reading on the socket circuit of 0.4-0.5A to 0.1-0.2A.
Put a few new 8 way extensions in while I was down under the desks which might have helped?
I did do an IR test on all the existing multi-way extension cables and all were fine on 500v.
The RCCB has yet to trip under normal conditions but as soon as I put a load on my benchtop AC to DC power supply, it trips.
Could be a fault with that bit of kit but that kit is not normally plugged in. I tested it at home and the RCD did not trip there.