I had a similar issue with our transformer at the farm.
My Combined Heat and Power (CHP) generator (56kw continuous 3 phase) was tripping off and recorded voltages of 255v and higher.
If you can show the DNO that they are supplying voltages over 253v, they are in breach of regulation and they HAVE to reduce it on the transformer tap. But they don't like doing it. After producing the data, they changed it in a week, but before that they weren’t interested.
Once it gets over 253v it can't be good for equipment onsite surely.
Our supply went over 260 VAC today, until I isolated it. After that I saw it hit 285 VAC at times. It seems a tree damaged a cable, almost certainly taking out the PEN conductor, causing the line voltage to float around all over the place, as well as pushing the PEN local voltage up to around 80 V above true local earth. This thread relates the tale: Open PEN faults . First time I've witnessed an open PEN fault first hand, and it's convinced me that the IET were right to make open PEN protection mandatory for any outlet installed to supply a car charge point.