British Summer Time came at the weekend.
My database server showed problems on the following Monday. When I investigated on the server, the time was correct when viewed in the bottom right hand corner, but when the event log was viewed it showed that (logged at the correct time) the 'system time' was an hour earlier.
The database server and the DC for the test environment are both Hyper V.
Eventually the problem was resolved by going into each of the test and infrastructure servers and in the Date and Time settings, unticking the DST tickbox, and manually setting the time to the time required.