why wont my megane start?

i have a renault megane 2001 1.6 16v. i cant get it to start? all lights are lighting up on dash and cd player is working so its not battery. i have changed spark plugs the other week. it cant be my starter motor as it starts when i have bump started it. it starts sometimes and then wont start at all. i have even changed the batterey in my key fob because i thought it was the immobiliser. but it has not resolved my problem. what else could it be?please help as i am broken down away from home and im missing out on starting my new job!thank you

Ryan Hope
May 2008