240 General: Wet carpet mystery

[146ken]

I know, I know, this is an old favourite but I've followed all previous advice on this topic and drawn a blank.
Briefly, early this week I had cause to check the fuses on my 240SE and found the passenger-side foot well was a bit damp, and in the rear foot-well was a good inch of water. Drivers side was dry as a bone. The water was clear and seemed fresh, that is things hadn't started to fester and smell damp. This morning I've stripped out the carpet and foam underlay and dried things out.
I then started pouring water round all the likely points of entry. The sill drain holes were good and clear, opened the plastic grommets on inside and the sill is clear of debris and blockages. I've checked all the rubber plugs in the foot well and they are intact and watertight. No leaks on sunroof, and the doors appears watertight.
The carpets were sodden at floor level but dry above which seems to point to a leak at or around floor level but I can't see any obvious entry point.
I've come to the conclusion that the gale force winds early in the week have forced the water in somewhere. The car was parked heading into the gale and torrential rain.
Any thoughts or pointers to what next would be well appreciated.