My Zanussi FJS1425 says E40 door open when not. Any ideas?

I have a Zanussi FJS1425 (Jetstream 1400). I close the door and set the wash options then click start. The door sounds like it locks like normal, but then tries again and comes up with the E40 error.

There's nothing stuck in the door and it's not over weighted.

Any ideas?? Thanks