It could be anything.
If the fuel pump is gone you will see a hole on the housing where the injector pump once was, but it is unlikely someone has taken it without you knowing and as it is coded to the ecu and key i very much doubt it.
Check there is fuel, check immobiliser , if the pump is still there then get a mate to tow you at approx 65 mph, then with ignition on and in 1st gear immediately lift off the clutch and turn key and it should start, however , if the van then only runs for a few moments and stops again you can assume the fuel injector pump is no good and will in hindsight cost more than the van is worth to replace.
Let us all know how you get on, and good luck .
Rayner diesels of Newbury, April 2009
