Have a look on http://www.brick-yard.co.uk/forum/ – dunno offhand which threads deal with limits/reclassification but there are a few – mebbe try searching the t4 forum first
As mentioned, I think you might manage to get it reclassified as a camper, doesn't help you now tho'.
>seems a bt of a grey area
In terms of working out what's what, yes, but I think it's pretty clear cut according to the registration document, IIRC it's either M1 (non commercial) or commercial (whatever that designation is)
>as a car-derived van(based on Golf and Touran),
Not sure it's as straightforward as that, might have the same chassis but the bodyshells different (I guess that could be the reasoning)