System time and date is set correctly (verified via `date` command), timezone is
lrwxrwxrwx 1 root root 33 Jan 1 12:15 /etc/localtime -> /usr/share/zoneinfo/Europe/London
The date and time kcmodule also shows the correct time and zone (Europe/London BST). The digital clock plasmoid shows one hour earlier, except between 12am and 1am, when it shows the correct time.
Please try to build and run the attached example in comment #14 in bug 335050 and post the result in that bug, thanks.
*** This bug has been marked as a duplicate of bug 335050 ***