2003 merc 311cdi problem starting?

i was wondering if anyone could help me out,i have a 2003 mercedes sprinter 311cdi,it has 90,000miles on the clock.i have owned the van for two years and had it regularly seviced in that time.Recently it has been struggling to start,at first i thought it was the battery and a new one helped a bit,but it is gradually getting worse again.the starter is turning and turning and it eventually goes but even once it's hot it still struggles.It has had new glow plugs fitted also and this has not helped,and the computer shows no faults!checked fuel system and it seems ok,but it did seem to start not long after service,could there be a problem with the fuel filter and are there different types?i have tried bleeding the fuel line and even fitted a hand pump to make sure it was getting fuel.Apart from that,once it starts it's 'normal',no smoke,noise,etc.i'm at a total loss and so is the garage,but i need the van for work and i just know it's getting to the won't go at all stage!!!any help/things to try would be most appreciated,thanx