Hi Jam,
You don't say what year your Bav was built so the design and interior fittings, cupboards etc., may have changed. On a number of older designs, the areas above and behind the cupboards were very difficult to get behind, but on my 36, and some of the 37s and 38s it was possible to remove a small horizontal panel in the tops of the cupboards located under the side decks by undoing a couple of screws. This would release a small thin piece of plywood which gave very limited access to the bolts or screws used to secure the rail stanchion bases and the otherwise void spaces beyond those cupboards. It would also allow you to hold a small camera up inside where using the camera you could then photograph the interior of those spaces which would otherwise be impossible to view and will help in determining where the water is leaking in and to where it tends to run. I've attached a photo or two showing the interior of one of those spaces and from which you can see the trail that a water leak had left. Good hunting, but then the problem becomes one of how do you fix it. In my experience "Captain Tolleys Creeping Crack Cure" works quite well once you have found the leak.