We had similar issues, the volts were over 250 because we were right next to the distribution board (incoming feed) and its sometimes necessary to be raised because of transmission / volt loss over the building (or at the sub depending on distance)- the other end of our building was lower than 220v!
Ask your facilities manager to check which electrical distribution board you're on - we were jointly on the kitchen one which had fluctuations everytime the fridges, cookers and the like came on which was a real pain.
We're on a seperate board now and although still some smart trimming there are much less power related events.
Edit: your PAT testers should be able to check your volts out when they do their annual checks