I've just logged 24 hours of data from my UPS, recording the input voltage every minute. For about 4 minutes it hit 258V, the lowest was 249V, and the average was 253.627V. The legal maximum is 253V.

I shall be logging a fault tomorrow with Western Power Distribution. I haven't got much else to do 😁

My UPS isn't a precise piece of equipment, but my previous UPS used to almost permanently run in voltage buck mode, so I'm reasonably confident my mains supply is over voltage.