I've sussed it thanks.
I did what a I should have done earlier, and read all the similar problem letters in the archive.
Thanks to the guy who rightly diagnosed that the small adjustable valve on the top of the tank with the black tube coming from it was clagged up with lime scale. It was on mine too!
I did have a problem getting the casing to split, even using the correct tools the screws just kept turning.
So I had to resort to beasting the sides apart and cutting through the plastic tubes that you access the screws through then taping it up after the repair.
Thanks again for your help and excellent site.
Bob Saunders, December 2007