how can i get my car to start please help!!?

I have a renault megane 1.5dci 2004 , it has been a really good car up untill the last month! i have just had a full service and cam belt change but i now have a problem with the starting of the car , it just wont seem to tick over ,well first thing in the morning it does but if i keep stoppin the car the more harder it is to start again, i have found out if i put the heaters on full heat in the car it seems to help it abit better but not fully , if anyone has any idea of what it could be please please help as it is driving me crazy and i need my car for my job , thanks !