Get your injectors tested for leakback rates - pound to a penny you have at least one with excessive leakback.
This prevents the common rail reaching the pressure to activate the ECU, hence the prolonged cranking required to build up the pressure (> 10,000 psi).
This test should cost you around £40 at an independent diesel specialist (Less than the cost of one one glow plug).
Don't get it done at a main agent, they will charge you more.
What you do next is well documented on this site
