Quick update, The system seems to have rectified itself (or learned as VW put it)
This week the vans covered approx 500 miles and the AdBlue is now showing 650 miles, so if anything it's improved on the range.
The van is still going in to VW on Monday, but just for a free refill, apparently the van has to plugged in while they refill the AdBlue and the data is sent to VW to be checked/analysed.