(A reply I wrote describing the problem hasn't appeared for some reason! I'll try again.)
The existing seal is between the inner top edge of the door and the inner edge of the frame (it's attached to the frame). There is no seal between the outer top edge of the door and the outer edge of the frame. Water gets in there and collects under the underneath of the top of the frame. Any disturbance and that water drops onto the top of the door which slopes inwards. While the door is closed the water then meets the seal and can't go any further. But when the door is opened, this water pours down the inside of the door - and there is a surprising amount!
I'll post here when I find a solution.
Yes I converted a Citroen Relay to a MoHo a while ago (see link in my signature) it wasn't really a problem as we rarely used the side door, but I did fit guttering to stop water getting in when we did leave the door open.
It is after all a goods vehicle so not a problem on the production line, not got a van handy to look at but I can't think how a P seal would help with held water, but it should pass down via the sides of the doors seals to the bottom and out, maybe a seal isn't fitted correctly as I suggested a while back Rob, maybe a worth a closer look, and perhaps tap them all on tighter.