The question is though, if this has been in place on the old heater all that
time how come its suddenly reversed? I'd imagine this is the mindset which
has caused the confusion. It might have been wrong for years but only showed
up due to a fault and some components in the new heater are causing it to
still show up, It can be very dangerous to assume that just cos it used to
be fine, that the wiring was always correct.

Brian

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The Sofa of Brian Gaff...

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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 :-)