A new valve costs £25. What I did was lever the coil off the valve and replace it with the coil off a cheap (£5) washing machine valve. It just pushes on and if you're lucky and get a good fit you can even do it without removing the shower or disconnecting the plumbing.
Owen, March 2008
electrofix of doccaster 01302 341177 cost me 5 pounds last week
keith goodwin, November 2006
I have the same problem. Seems to be the mechanical side of the valve sticking. Taking it off and manually opening/closing it seems to help but I don't know for how long. When these things start to stick they are better off being replaced. It's a sealed unit so I can't fix it properly. Did you find anywhere to get a replacement?
Michael, September 2006
edit
I can answer this question...
Where can I get a new Solinoid Valve? Aqua-Tronic W9500s slimline?