Thinking it over and over again now, the strongest possibility that came to mind was the heater matrix. Could it possibily be leaking from one side as to cause only the drivers side to be wet?

Though when i was with the car i did think the water could be coming in from the floor, hence why we couldnt find any sign of it coming from above. But where is the sealing plug in the floor? And the last 2 times he's taken it out it has been lashing it down quite heavily. I did have a look under the car but couldnt really see anywhere where it could be coming from, but then i didnt realise there was a plug down there.

Thanks or your suggestions so far chaps.