Paul,
Sorry, but I never had a close read of your comments in your above post.
Am I assuming that a system of different boards for different Rates applies, in the UK?.
I see that the Consumer unit above the Service Fuse has a large cable exitting it, would this be for a Shower?.
Over here, we would normally supply a Night-store heater or Hot water cylinder from the same board, but it is normally fed from the Meterboard(outside, usually) via the Ripple Relay+ Pilot Wire.
As a rule, an MCB, is fitted to the left side of the Main Switch or a Multi- DIN Rail Board is fitted and all of the Night Rate equipment has it's own Rail.
This just keeps everything tidy and makes Fault-finding easier. [Linked Image]