How can I stop all the programmes on my zanussi dishwasher sticking?

Hi there....HELP please...my zanussi model DE4554S has recently stopped completing full programmes. It will start and will dispense the soap but then gets stuck. If left to its own devices it will happily wash the dishes all day without ever completing the cycle!!!
When I eventually open the dishwasher there is dirty water stuck in the bottom of the washer and the dishes are unfortunately clean! How annoying!!
I've tried it on all programmes and the same happens. My dad who is also a plumber has checked the drainage and all pipes are clear. ANY HELP/ SUGGESTIONS PLEASE???!!!!