I have discovered a small leak in the boot, and looking at threads here, so have many others!
I've taken on board the main possible culprits, aerial base, window seal, boot seal etc, and I think mine may be coming from the boot seal join by the boot lock.
I took all the carpet out and saw that there was a lot of condensation on the inside of the boot lid, which explained why that piece of carpet had lots of mould growing on it, it had clearly been an issue for a while.
Once it was all dry, I drove for a few hours on the motorway in the rain going back to work, and when I got here, the boot was still dry; but having left it standing in the rain a few hours, I went for a short drive, and when I got home, I noticed there was a small amount of water leaking into the centre of the boot from the door lock / boot seal area.
Judging by the small amount of silt that was around the boot shut, which pointed into the boot, it looks like rain pools in the boot gutter, and when you drive it then forces it around, and in my case, then finds itself leaking into the boot.
That strikes me as a bit odd, as there is a metal lip all around the seal, so I thought maybe it was leaking in through the badge / key lock, but that area seemed dry.
Does anyone have any experience of a leak in the centre? I'll be putting some sealant on and under the seal join in the next few days, (when I get to the shop to buy some) to see if that is the issue, but if anyone has any other pointers for that area, or if its a symptom of the boot lid not being seated correctly, then let me know and I'll add it to the list of things to try!