Thanks for the reply. Doesn't seem right that they didn't put a piece of hardboard in.
Of course there's a back story, as it was my daughters 500, and other than servicing (which I insisted she got) the car was neglected.
When I took the carpet out today it was soaking wet, and there was 3cm of water in the wheel well.
I asked her what was going on, and she told me that an almost full 5ltr bottle of screen cleaner had leaked (she didn't do the top up tight). I've had the carpet in the sun all day, and it's nearly dry. I took it for a jet wash to test the boot seal, and it was fine, no leaks at all. Her throwaway comment about the liquid in the boot, "hadn't it dried out?". Now I know why there was so much condensation in the car.