My 54reg megane struggles to start?

my 54 reg megane used to start ok. i had the cam belt changed and now when i try to start it in the morning it starts ok however once i apply the accelarator it tries to cut out and makes a knocking noise. It then starts ok through the day. it only happens when the car has sat still for hours. Its driving me mad. The garage don;t have a clue

tracey
November 2009