Okay, I checked it today and the maximum I saw on the meter (I didn't watch
it all day!) was 58A
I expect when the kettle, fridge, the three water coolers/heaters, all the
lights and all the PC's are on this figure would be greater.

The Meter does state Max 40A

There doesn't seem to be any RCD's on the main (2) consumer units, just
MCB's and a 100A isolator on each CU.
There is a third very small CU with a main switch and two fuses, this is
marked "Top Floor"
No idea if this then connects to a newer CU or not - didn't get time to
check!
All the CU's connect to the meter via a Henley block

At lockup time, with most of the lights off, there was about 10A being
consumed.

During the day I put my clamp meter around the main earth (the cable that
comes from an earthing block where all the earths from the CU, Gas pipe, etc
connect to, to the main incoming supply cable)
This was showing between 800mA and 1A - should we evacuate the building!

Sparks...