22 seems terrible, unless you are a delivery driver, leaving the engine idling for long periods of time. Does your trip counter count the miles properly? Satnav a route and see if it records the distance correctly. Of course the other thing could be that whatever measures fuel flow is dodgy. Could also be a software fault maybe. I'm not a mechanic, but I think if your engine was genuinely getting half the MPG in a setting where it should be getting 45-50, there would be some serious indicators (black smoke exhaust fumes from unburnt fuel or something!).
When you say it should be near 50, is that from you measuring fill-to-fill and calculating, or just your expectations of the van? (Which would be reasonable for a diesel NV with some longer journeys).
A custom remap from a good place (I used AMD Tuning in Essex), should reveal any faults, as they do a health-check before they begin. So far have noticed MPG improved on country lanes as I no longer have to down-shift on hills, keeping the revs under 2000rpm.