I've started having a similar problem, but on US Central time, not BST. Instead of jumping a hour ahead, mine is jumping 5 ahead. It is not anything to do with the BIOS time, because that remains correct. I don't think that it has anything to do with the CMOS battery, because if that was too low, the time would fall behind, rather than ahead.
It seems that it is always only the hour that gets wrong, because the minutes and seconds are right on target. I changed the time server again, just in case the one that I was using went haywire, but I seriously doubt that is the case. It seems to me that it is something in the OS itself that is not right.
EDIT: Considering the difference between BST and US Central DST versus UTC, it seems that the OS is trying to run on the latter, rather than the zone on which it is set.