Thanks - I take it it's worth getting inside the machine and seeing if anything's loose in the door lock area then. Is there anything in particular to look out for?
This is an intermittent problem. Sometimes the machine just won't start - I've tried holding the button in for a long time, pressing the door with my knee and all sorts. Eventually it will start and after that it works fine, but there's no way to predict how long it's going to play silly...