If you are sure the wiring is good, it may be worth hiring a power logger to record the supply voltage over time - it is not always safe to assume the DNO side is perfect.

Mike.

The only one of those which is fixed wiring is the lighting. MK used to be excellent, but their reputation has declined in recent years. The obvious answer is to replace it.

Is surge protection installed, could be voltage spikes from machinery in other industrial units.

Thank you for the swift response. I was told that might be the case, can this not be kicked back to the DNO to test? 

Thank you for the quick reply. There seems to be no typical time the faults happen. However, if the lights are increasing, it is generally at night over the weekend. The area is under redevelopment, which could create problems. 

Hi Chris, thanks for speedy response. I agree with that statement; I swear by MK, especially regarding dimmers; however, they seem to be as bad as the cheaper brands now. I have been looking at using Click as my supplier swears by them. 

The term "surge" tends to get over (or mis-) used. Surges (of the kind you'd expect a surge protection device (SPD) to help with) are very short duration and usually very high voltage - typically measured in kV and micro-seconds. Usually the results of lightning or large switching events. If someone can see lights visibly changing (and not exploding) you're probably looking at something of far longer duration and lower voltage.

There are a number of things that could contribute to that - perhaps the most common is a loose neutral in the 3-phase section of the distribution system (i.e. in the DNO section if your supply is all single phase from the origin) . With the neutral connection lost single phase loads are in effect connected in twos in series across 400V and can see anything from 0V to 400V instead of 230V - all depending on what happens to be switched on on the other single phase loads at any particular moment.

You would need some evidence to present to the DNO though. If you can show voltage readings out of the normal range (216.2 to 253V) or L-N loop impedances out of range or rapidly fluctuating, that would be enough to call them out I reckon.

   - Andy.

Check the tightness of all neutral connections back at the DB including the neutral link. 

Did you install surge protection?

Put a clamp meter on all the bonds to extraneous conductive parts and see how much current is flowing in them, if you have more than an amp or 2 then you may have diverted neutral currents. Check the voltages between the 3 lines to earth and the 3 lines to neutral and neutral to earth,anything strange? If you have either of the listed anomalies time to call the DNO on 105.

Oops . Just seen it was a single phase supply.So voltages neutral to earth and line to neutral.

JP

But it will be 3 phase back in the street - my concern is that a similar problem could be happening on the supply side.  However the other thing to check is that some lights not on  dimmers are also affected.

Mike.

The DNO  will normally only act if you can give them something more than anecdotal evidence, but if the effect does not occur to order, then that can be tricky. Leaving a logger plugged in, which is essentially a volt  meter that records its reading over time is easier than hanging about waiting for an intermittent fault - bit like the mystery noise in the car that never occurs when you have the man from the garage in the passenger seat ;-) 
But it is also really important to rule out the dimmers, are there some non-dimmed lights and do they flicker too ?
Mike.