Their voltage readings are much lower than the worst case you've observed, but still clearly out of spec. I wouldn't bother phoning them, it's pretty useless, and I wouldn't accept any phone calls either, insist on having everything in writing.
I would write directly to the Supply Quality Manager whose name and address you have, reference the letter, their previous acceptance that the supply is out of tolerance, your continued problems, etc. Give them a reasonable but clear deadline, say 4 weeks, and state that you will be in contact with the ombudsman if you don't receive a reasonable response in that timeframe. They will probably ignore it or send you some standard template letter, but that's fine, this is all good evidence. A recent letter and evidence of recent inaction will be helpful in getting the ombudsman on side.
Unfortunately the ombudsman is probably more used to dealing with complaints about money, payment to metering operators, etc. and may not be too familiar with technical supply-quality complaints. Still your best route though.
Again they said I am the last house on this phase, so surely this would mean all the nearer houses are getting higher voltages than me and I am the highest.
It seems you do not understand three phase supply.
Since you are the last one, you are going to experience the highest voltage across neutral (or should I say PEN as this is TN-C-S). This neutral voltage might come from the load on:
- only your-phase (you experience voltage drop in socket) or
- only not-your-phases (you experience voltage raise in socket)
- or some other combination of loads where the end result is more convoluted.
The point here is that neutral is shared between all of your neighbours. Had you been connected next to transformer, there would have been no problems with overvoltage. IMHO.
Put another way, being far away is bad because both line and neutral voltage are more
variable and with the particular combinations of loads on your street this variation results in your seeing high voltages. Typically, your line voltage may be lower than near the transformer (relative to N
at the substation) but almost certainly your neutral voltage is a long way from zero (again relative to the substation N). In your case the neutral voltage is perhaps a few tens of Volts, but opposite in phase to your line voltage because the neutral is being pulled away from zero by loads on other phases, that increases the L-N voltage you observe.
The most recent information you've given suggests that you probably have a full-sized neutral (based on the date) but that your phase is very lightly loaded whilst the others are heavily loaded at night and without any large loads on your phase this results in the long, relatively high resistance neutral being pulled away from your phase towards the other two. Maybe there's simply an unlucky combination of loads and it just happens that most of the houses on your phase are unoccupied, gas-heated, etc. and most of the houses on the other two phases are heavy night-time users with storage heaters. Statistically this has to happen occasionally. Or maybe the cable jointers screwed up and you are the only house on your phase.
It would be possible to make some further measurements that might help confirm what's happening, but it's not especially useful, it's UKPN's problem to fix.