Bumpity to this, I had a good clean of the car yesterday and investigated and it seems the source of the water is in the two conduits carrying electrical cables from the vehicle to the hatch - they should be sealed securely to the vehicle but both of mine were flapping about. Water flows in through there and then into the spare wheel well!
I've secured one but on the other one the tab seems to be broken :S not sure how to fix it, but I might just apply a bead of silicon to seal them both.
...left the wire brush at work (I'm using it on my Karmann, which is stored there) and so didn't get a chance to clean it up; will do it next week I guess.