Prewash light flashing on Hotpoint Aquarius WMA54?

My Hotpoint Aquarius WMA54 needed the filter cleaning so following instruction i located the rubber pipe underneath and the filter section (containing the ball) and cleaned it. After re-assembling i now have a new problem. The PREWASH light keeps flashing and the washing program stops.
The user manual says the PREWASH light indicates that there is a blockage or kink in the water inlet pipe, however i have checked this and there isn't. Also, water IS getting in, but it appears to be coming straight OUT again.
Any suggestions as to what is causing this would be appreciated.