My 1.6 Focus Sport has had the same problem a few times since i got it 3 months ago. I came to realise that if I try to turn on the engine by turning the key round all in one go the engine will sometimes fail to start, especially if the fuel tank is showing less than a quater full :? It seems that if you turn the key to the second position, then leave it until the fuel gauge needle has found it's position (usually in the amount of time it takes to put your seatbelt on), then turn the key to start the engine it will work every time. Hope this helps if it hasn't already been said before :