Soooo many rumours...

Churtz is correct. There is a buletin from the manufacturer. The first step is to check the ecu calibration is correct. Most are anyway.

Then its almost certain to be injector issue. Doing a leak off test wont always show up a fault. Nor will sending them away to be tested.

Also, on the above comments about fuel rails, they have a mechanical valve in the end of the rails, just a spring and washer, and these weaken and allow the fuel to drain straight back to the tank.

I specialise in these vans, its quite common for us to get other garages sending us their vans to fix as they cant. Most garages seem to replace items willy nilly and still not fix them.