Following my last post I tried measuring my voltage with more high power
devices attached (both showers, over, microwave & kettle) and this time the
voltage dropped to around 215-216V.

I rang EDF energy again and they sent someone out yesterday afternoon.
Strangley all he did was disconnect the main fuse and measure the voltage
coming in to the meter, which was 247V. He drew a conclusion from this that
their line was OK and it must be my wiring!! When I pointed out that when I
measured the voltage with minimal load it showed 247V too, and that it was
only when under load that the voltage dropped, he basically shrugged his
shoulders. The only suggestion he could make was that the high and low
power devices must be running off the same MCB, but I've already confirmed
this is not the case.

I'm not sure what they're going to do next, but it basically seems as though
some link between the substation and my internal wiring has higher
resistance. Earlier postings from others seem to suggest it's the supply to
my house, but I guess I should try to test the resistance between the main
isolator and my consumer unit. Does this sound like a reasonable idea?

Cheers,
Chris