Thanks for all the input.

On meter accuracy the KT65DL measures voltage to  4 digits, example 230.5, so my max error is 4.6V + 0.4V, total 5V.

Interesting to see that the meter linked above is much less accurate for AC voltages than DC, I guess it's difficult to compensate for voltage drops across diodes, assuming meters use a bridge rectifier.

Meter appears to be more accurate than that in real life, the error becomes a concern when we are on edge of acceptable, if I am seeing 216V, is it actually 216 or 214 for example.
So the voltage variations I was seeing were occurring around about 5.30 in the evening. It's in a village that is fairly long and strung out, with a mixture of overhead and underground lines. Looking at the SSEN network map, the substation is about 4km away, marked as 33/11KV, which I assume is input and output. Also states fully constrained by thermal on the 33KV input line, no constraint down stream. There is also a solar farm covering a few hectares presumably on that substation. Looks like the substation can take a 50% reverse feed.
I think the transformer for the village is about 1km away and the house in question is close to the end of line; with a couple of houses another 0.5 km outside the village, may be on a different feed. Main part of the village is spread out over about 1Km. Haven't managed to find a diagram of the local network

Customer thinks he has actually solved his charging issues with a new 12V battery in the car, but he was reporting issues with both cars, so not sure. I am also seeing quite big swings in my house, 5 or 6 houses down from them. Now looking at this out of interest as much as anything else and wondering if there is actually a bit of an issue with the neutral. I am going to take some measurements of neutral voltage over the weekend to see what I get, will use a different meter as the KT65DL doesn't measure voltages below 25V.

I shall put my money on a network problem rather than a metering one.

That said, I fully appreciate GK's comment about the type of meter which is required. The DNOs must have them.

The real question must be where the burden of proof lies. Does a customer have to show that a supply is problematic, or does the DNO have to show that it is sound? At first glance, ESQCR 2002 is unhelpful.

I shall put my money on a network problem rather than a metering one.

That said, I fully appreciate GK's comment about the type of meter which is required. The DNOs must have them.

The real question must be where the burden of proof lies. Does a customer have to show that a supply is problematic, or does the DNO have to show that it is sound? At first glance, ESQCR 2002 is unhelpful.