I found it got worse over time, and more and more channels had no signal being received.
In the end, I hired a ladder and spent literally hours at the top of it realigning the dish (not so fun when you don't like heights). No success, so I eventually came accross the following thread on digitalspy
http://www.digitalspy.co.uk/forums/showthread.php?t=727185
Lots more info about the problem.