Mercedes ML270 not starting?

Hi my ML270 will not start. I had the battery replaced and that worked for a couple of weeks and then it wouldn't start again. I have had it into the garage and they have replaced 3 fuel pipes and the car started again beautifully - only for 2 days though! I had it running for an hour today and when I went back to it 5 mins later it wouldn't start and I had to leave it where it was as it wouldn't start off jump leads either. Can anyone help?