I thought that it may be helpful to anyone having the same issue that I had, namely a leaking rear tailgate or a leak somewhere. I was getting a lot of rain water getting into the spare wheel well, and I couldn't figure out where it was coming from.
I took all the trim, carpets and fittings out of the boot area and left them out for a couple of months, checking after each time it rained to see if any rain had got back in. Each time it rained there was some water still getting in. I carefully cleaned off everything that I could get at in the spare wheel well floor and body work. I used a hair dryer to make sure that the hollow panels were dry, and then I took out things like the tailgate flap and stuffed Vaseline into the screwholes.
I checked and cleaned the sunroof tubes, and made sure that the pipes through the rear arches were not blocked. I also readjusted the body edge, as per the service advice, and took off most off the tailgate seal and reseated it using Vaseline under the seal. The seal appeared to have deformed at the two lower corners but otherwise looked in good condition.
This is the point at which I can image that people start to pull their hair out, and look, as I did, for anything at all that could be a problem - back bumper fixings, rear wiper, rear wiper washer pipe, etc, etc.
I sat and thought about what could be happening, and out of frustration I decided to put my hand in my pocket and buy a new seal - £120 or so with delivery.
And hey, the most obvious answer turns out to be the solution. I used a windscreen non-setting black mastic to put a fine film of seal under the tailgate seal and refitted the new item.I have had the vehicle sitting out in very heavy rain for the last two months and [fingers still crossed] not a drop of rain water.
I think that we all do the obvious and look for a leak somewhere and decide that it must be somewhere unobvious !! In fairness I could have saved a couple of months of sponging up the rain water every time it rained, but I also think that I was reluctant to spent £££'s without being certain.
But I think that my advice now would be - if the leak is not the sunroof then really, really consider just fitting a new tailgate seal.
One thing that I haven't sorted out is water getting in when I drive through a lot of big puddles in heavy rain. I have no problem in heavy rain on a good road surface, but if I drive through a lot of large puddles on the side of the road then water gets in somewhere. I may just get the rear underside undersealed, and see if that sorts it out. At the moment, I just avoid big puddles !!!