Firstly, see if a few squeezes of the fuel primer bulb (at the side of the engine, on top) enable it to start. If so, then you may have an air leak in the fuel system, allowing fuel to run back into the tank when the engine is switched off. Firstly, check all the thin rubber fuel return pipes between the injectors. It might be worth buying a length of this pipe and replacing all these anyway. Also, the end injector has a closed 'stopper' instead of a tube to the next injector. This was the problem for me, as it was split. Replacing this for a few £ was the answer.
Alex, September 2007