A little bit of extra info, not sure if everyone will have this problem - I noticed some leakage occuring from the small clip that sits near the rear window, holding the sail panel in place. Although it doesn't go directly into the boot, it is possible that some could in theory leak in. If not, it'll just run down the empty section at the side of the car (where the electric aerial is etc.). It might be a good idea to remove it (almost impossible without breaking I think!) and replace it, or bung some sealant around it without removing it (remember to dry it thoroughly first though and make sure it is fully sealed all around and at the bottom of the clip so no water can get trapped in that area). Chances are that when removing the sail panels, you may possibly dislodge it. It's the hole at the back of this pic with grey primer around the edges (paint came off unfortunately)
Like I said, it's just something I happened to notice. It's a bit easier having no boot interior, you can just reach around till you find the water.