Right. Well, I removed the light cluster, tried to find a leaky spot. No luck. Put the light back in so I could spray the car with more water and... nothing. Not a drop and loads of water on it. I really really really hope that may be it for the boot.
However, there is also water in the front passenger foot well which, from the link posted about, is probably the pollen filter leaking. Another thing to fix.
I'm still waiting for some proper rainy time but there have been a couple of days of heavy rain this week and we seem to only have about a tea spoon of water in the boot now. Much better than the pond we used to have. No horrible damp smell in there now either.