I'm afraid the need for a sign warning you of the presence of the camera is not a legal requirement and the failure of the camera operator to display a sign does not mean that the evidence from the camera of you speeding is inadmissable in court.

If you can be bothered, have a look at the forum on Pepipoo for the full low down:

http://www.pepipoo.com/NewForums2/index.php

I got zapped the other week and the guys on Pepipoo have been a real help. See my post here:

http://www.pepipoo.com/NewForums2/viewtopic.php?t=2079