Have you checked your starter motor and battery? I've has the same problem and have had both changed and so far so good. Also, have you check your immobiliser like graham suggested? Mine wouldn't start again two weeks ago, and it ended up being the immobiliser. The central locking wouldn't work, so the immobilier remained active, so the car wouldn't start when opened with the key. This ended up being the central locking fuse, as it had blown. But it's worth a look.
Kellie
February 2005