here you go, cut and paste of an answer I gave on LRonly.com last week:
Had this issue when I first got my discovery with the OEM ladder already on. I too tought it was the ''thickness'' of the ladder that was allowing water to go in. Went to the dealer, and the service manager who is also a member of the local club, showed me how to resolve this.
You first have to open the door all the way, and while a second person is holding it and not allowing it to move, you have to loosen the upper hinge. Then the second person lifts the wide open door a little to allow the hinge to move in further than it originally was. thighten the upper hinge, and this will create a thighter seal.
Worked for me.