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The water is probably getting in through the air vents, see this thread on the Pommy forum and especially post #3: Fixing the water in the boot problem
Happy to be proven wrong here, but having used this really useful thread to identify possible causes of the leak into my spare wheel well, and fixes for same, if the vents were to blame most of the water ingress would end up in the 'v' between the outward sloping boot floor and the rear wing. What I did find was the large grommet beneath the right side lamp cluster out of its orifice and what appeared to be water tracks in the dust leading from them. I am assuming it was disturbed when the tow-bar was fitted. Hopefully - having replaced it (without any 'gunk') - the leak will cease.
Edit to add that the water can be seen between the battery tray and the big plastic sub-woofer echo chamber thingy. A torch helps. Small amounts can be mopped out with a towel pushed into the gap with a suitable stick. I have had to bale mine out with a sponge on several occasions which entails removing the spare wheel and the sub-woofer box = PITA.
I had this problem and I think I found where it's coming from. I sprayed the back of my car for a while then looked to see water coming through one of these white clips. I'll try silicone it to fix. (I assume it should be sealed from the other side but I'm not taking the bumper off lol) Here are some pics. Hope it helps.