How can i fix my Clio 197 57 reg?

I have a Renault Clio 197 57 reg, a few times it has had trouble starting and after leaving it for a few minutes it has then started and been fine for a couple of days. The other morning it wouldn't start at all, and has refused to since. AA were called out and said it was the crank shaft sensor, this has now been replaced and it still doesn't start. I have also had a mechanic look at it but their computer does not register any faults with the car and they are clueless too!
Can someone help?
Thanks

Chris
February 2011