How do I fix my Zanussi jet system 1200?

Our Zanussi jet system 1200 will not start the washing. When the machine is full, ready to wash, door closed, program selected - the interlock light on the door will not come on and the sequence will not start. Mains power indicated on the front of the machine. I've changed the selected program but no changes. I've had the top off and could find no obvious physical problems