Rainwater in spare wheel well

Hi
I have had my 508 saloon for over a year now and have noticed that the well holding the spare tyre gets up to a couple of inches of water in it after heavy rainfall. The cover is dry and so is most of the rest of the boot except for behind the wheel arches and after pulling back the boot lining I found small wells that were also full of water. The mechanics at the dealership where I bought the car suggested that it was caused by water entering the boot when I opened it during or after rain, I had found this a problem so agreed to remedial work being done (the fitting of a kit designed to combat this problem). The problem persists and it is now obvious that the solution lies elsewhere. Have other Peugeot owners had the same as trouble? And if so have you found a resolution? Any suggestions as to the cause or the remedy would be much appreciated.