They mentioned an investigation charge when I first booked the van in. I picked the van back up yesterday and they told me that their investigation revealed no leaks... so I'm guessing they just blast it for 10 minutes with a hose pipe. Luckily I had taken photos of the leaking fabric, printed them off and took in with the van. I spoke to their service manager, Neil, and he was very understanding. I told him that the leak only starts after 2-3 hours of rain, and just wetting it with a hosepipe or even pressure washer does not cause the fabric to leak.
He has stated that he will make contact with VW, send them my photos and see what they say about it. (VW must know there is a problem by now, so many people seem to be affected).
I wasn't charged the investigation fee..
So my advice is to take photos of the fabric leaking if you can, and take them with you and ask to speak to Neil, who now knows all about this.
Good luck and please let us know how you get on.