I've rotated my screen using xrandr -o left to use it in portrait mode and, to my pleasant surprise, the oclock now works as it always used to. There doesn't appear to have been an upgrade to x11-apps since I reported the bug, but it has been some weeks since I last rebooted or even restarted my X server, so it's possible something else changed in the mean time. Either that or xrandr did some magic that overcame whatever was breaking it; or the change in screen position (it sits in the top right of the screen, which now has a much lower x co-ordinate) was relevant.
Eddy. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]