soaked offside floor

Having had the same problem, you have my sympathy (although my G does not have aircon, so that was one possibility that I didn't have to rule out).

Here is my take (but I'm not that technical so bow to better advice elsewhere):

You could just start replacing things such as seals until problem stops but if you have a bit of time then I think that first thing to do is find out more exactly where water is getting in.

Now that you've removed mats, sound deadening material etc. roll up some newspaper into long sausages and wedge all around inside of footwell preferably just above floor level. Sit in drivers seat (assuming yours is a RHD and this is the side that's getting wet) and have someone outside give that side and roof a good soaking with a hose.

Which part of the newspaper is getting wet?

If it's the door side then likelihood is that it's the sunroof seal and/or drain that is blocked or water coming down inside of door (see http://www.gwoa.co.uk/content/water-footwell)

Clearing the sunrood drain is a fairly simple job (I used heavy duty strimmer line and lots of soapy water). Replacing the sunroof seal is also a fairly simple job (new seal cost me £77) and also resulted in much reduced road noise.

If no newspaper is getting wet then:

Fill up water bottle and run rear windscreen washer a lot and see if that results in wet newspaper.

Run aircon and see if if that results in wet newspaper (although I don't know where condenser drain outlet is located).

Not sure about leaking heater/ pipe connections. If that was the problem then presumably you'd have noticed having to top up radiator by now.

If none of those work then try driving along when the roads are wet - maybe water is being thrown up underneath and getting in that way (in which case, I'm sorry but I'm not much help)

Finally, start drying out those mats/sound deadening material whilst we've got some warm weather - it takes forever!

Good luck - and let us know how you get on.

 Look at the post "Water in Footwell"

I think you'll find the water is coming from the weather seal plastic sheet behind the door card.

Remove the door-sill strips and you'll probably find that the fabric material on the door seal trim is wet.

I had a similar problem on my 300gd 460, the water was coming in through the throttle end stop which is a plastic bung that goes through the body into the front right wheel arch. it might be an idea to lift the carpet and have a look, or thoroughly clean the wheel arch and check the state of the metal around the bung.

Yes - my many thanks to Arnie also on this subject.

Having done the job but for some reason having continued to think about the problem since, my only question is: is there a possibility of  the rainwater coming down the glass, onto the polythene barrier then down onto the inside of the door and just sitting on top of the silicone sealant? If so then there is nowhere for this water to escape to and long term could this not cause corrosion between the sealant and the door cavity opening. 

Maybe I'm just being paranoid.

I opted for Arnie's suggestion to add an additional plastic sheet attached to the top of the cavity opening that drops down into the cavity below the opening. I'm hopeful that this means there is no possibility of any water going anywhere other than down to the drain holes.