Do you have any more detail? Is it on one side? In the middle? How far up? This will be sketchy as we don't have much detail but...
The best thing to do is to sit inside the Doblo whilst you get a friend to pressurewash or hosepipe the top of the windscreen area so that you can see where it's coming in.
It's most likely to be the top of the windscreen seal, as I believe even the high roof version of the roof is in one piece so shouldn't leak unless any holes have been made or damage been done to it.
Other places to look are around the ariel, water could be seeping in around the seal, and then running down the front of the roof panel and near the parcel shelf making you think that it's leaking there.