LB, has the fluid changed consistancy in the reservoir? Mine is a little tight at the moment, fine when running but a little bit of a pain when your parking!
Have you drained the whole system and then refilled?
As I have a vented bonnet I think that some water may have got in, and perhaps when I have jetwashed the engine bay!
Mines off to Abbey to get flushed out and replaced soon so will let you know the story when I have it back and what they have found.