On Tuesday, 19 December 2017 17:44:05 UTC, Steve Poulton wrote:
The electricians have tried swapping the various supplies to the various
heaters around in the E7 fuse board

Whoever those 5 individuals were, they weren't electricians. An electrician would test the heater, circuit, and MCB using a calibrated test meter. Swappin things around and hoping they don't go bang is not a suitable test methodology [1]

I'm going to guess a phase-earth fault on the heater circuit and a reversed phase-neutral incomer on the daytime board (or vice versa) so the fault presents as a neutral-earth on the daytime board, which does not trip the MCB (but would trip an RCD).

Simple polarity testing at the various locations would rule that out.

Owain

[1] For electricians, it works fine in computer repair :-)