Bosch Classixx fills and drains continuously?

Hello,
I've just bought a 2nd hand dishwasher in a student house, and after the first 4 weeks (warranty now gone) its stopped working) It switches on fine, fills with a bit of water, then the drainage pump kicks on. If left alone it does this for well over an hour, and still doesnt start to wash!
I'm in BIG trouble if I cant fix this, can anyone help (Please provide idiot instructions if you can, as am new to this kind of thing)
THanks