Renault Clio emissions? ecu? injectors? help.

During November/December of last year occasionally my car would shake and shudder when it started. As the winter progressed so did the problem. It then came to the point where it would not start at all, it would turn over but only until the point where the battery was flat (sometimes started once in a blue moon). Tow starting the car worked every time but black plooms of smoke kicked out the back. Once tow started it usually runs for a few days but reverts back to its old ways. I put up with this for only a couple of weeks before enough was enough. I had the car diagnosis test done and the result showed a lack of combustion in the injectors. Is it possible that all of the injectors could fail at once? It was also checked for iron filings and came back negative. I was told to try the pump under the bonnet and pump it until it was hard to push fuel through. Upon doing this I noticed that there was some air in the Fuel lines. I have had a one way vale put in, the fuel filter changed, fuel lines replaced (Not to a great standard and tightened on with jubilee clips). Could it be the EGR valve? How could air possibly be getting in my fuel lines and would this prevent the car firing up and cause black smoke when it does?

Any ideas? This is driving me nuts!

Joe