fred wrote:

It's three phase in the street and they rotate the phases through the
street 1-2-3 to balance the load so the neighbour is likely on a
different phase.

The neighbour 3 doors down will likely have the same fault.

AIUI the phasing usually goes L1-L2-L3-L3-L2-L1-etc., so the pattern
isn't as simple as you suggest. And that's for new estates where all
the services are single-phase and have been connected up at the same
time. In older urban areas you're more likely to have a mixture of 1-ph
and 3-ph connections with different properties having been connected at
different times, so the phasing might best be described as
pseudo-random, notionally balanced...

Don't know the likely cause but as it's not load dependent it's not
likely to by a high resistance joint anywhere near him. Maybe one of the
ex-distribution network regulars will have a better idea but it's def
one to report to the local regional supplier tomorrow.

Don't wait that long, it's a potentially dangerous situation - report it
right away.

--
Andy