Not sure if you have fixed this now but I noticed last year I also had standing water in the well for the spare tyre, I looked online and found that theres a rubber bung under the car near one of the rear wheels and if you push this in you will see lots of water spill out on to the road, when I did this it fixed the problem for me, was a while a go so can't remember the details but some blockage occurs and then water ends up diverted to the boot well, lots of videos on YouTube on how to do it and it's really simple, worth trying before paying for anything at least