Had same problem.
It's the diaphram that controls the water flow. I purchased a service kit and it cured my problem. To check if this is your problem, drop the front, turn on the water and check if the small pin protrudes and makes contact with the switch. (you will see what I am refering to).
http://www.ezypart.co.uk/parts/part_id__2262
Ken Shier, March 2008