Symptoms - water in boot (particularly in the tyre jack well). Condensation on boot lid. Permanently steamed up rear window (never used to be a problem). Wet carpet on drivers side.
Firstly, if you come across any problems with water in the car or condensation on the boot lid its quite likely the problem hasn’t just started. It’s most likely been letting in water for ages. What you’re seeing now is the water that can no longer be absorbed by the carpet on the rear shelf - I just squeezed out what must have been the best part of a litre of water from the fuzzy carpet stuff on the rear shelf. Its obviously been leaking for a while.
Solution:
1) Cleared the drain holes behind the seatbelt posts. Cleared out some muck but water still getting in.
2) Ran a hose onto the roof and looked for drips into the boot. Small drip up near the petrol tank - On the passenger side there is a gap at the top of the boot wall. It looks like the suspension top mount is in there.
Also water leaking in from the light cluster. Rubber seal perished - New one ordered but temprary fix with sealant seems to have cured it.
3) Undid the carpet from around the rear window and with the hose still on the roof there was water running in from behind the rain rail.
This was actually really easy to fix. Pulled out all the plastic clips that hold the carpet in place and pulled it back. This exposes all of the set plates (3 in total).
The set plates are held in place by a load of 10mm bolts. I took off all the plates and could now fold down the front of the rain rail - exposing the water problem. The plastic around around the mounting holes had cracked and it looks like this rain rail had a) been fitted to another car or b) had numerous push out holes for fixing to cars with different bolt spacings. The blanks had been pushed out in 2 places leaving 2 dirty great holes in the rail next to the bolts or c) the previous owner made a mess of fitting it.
I cleaned and dried everything, put duct tape over all the holes (fantastic stuff and if you’ve never used it before it can jst about hold your car together if needed), made sure the roof was inside the rail, folded the front of the rail back up and put the set plates back on.