Ihave a 55 plate Ford Focus?

In March this year i purchased a 1.6 Ford Focus on a 55 plate from a well respected local car sales. They gave me a 3 month warranty. Since purchasing the car I have problems with the car on 4 occassions not starting being placed on a diagnostic machine only to show no fault. They have been helpful at the car sales but say it is not their problem as out of warranty. I also have had the power sterring fail on me. What could be the fault re not starting on occassions, the engine turns but appears to not want to fire. Secondly do I have any comeback on the car sales premises.