In article ,
Andrew Mawson wrote:

As others have said, with this sort of fault you will have
excessive light flickering, as will the other house you mention
on the same transformer. It's probably worth seeing what effect
your welder has on their lights, and maybe having a joint
approach to the supplier.

AFAIR, I thought that FiL made a mistake in
using the words 'motor mechanic', 'welding',
and 'flickering lights'. The electricity Co
were not interested in replacing the transformer.
I suppose an intermittant load, such as welding,
would not generate enough revenue to cover the
cost of installing an uprated transformer.

The OP does have a case here though because the
voltage is below 230 -6% with only a 12KW load.

It would be useful for him to check the voltage
when both houses are taking no load. Anywhere
near 250V would be ammunition to argue *against*
their simple getout of tapping up.

--
Tony Williams.