Thank you pric4443 and Andy for your replies.
I have had the blocked drain pipes before and I cleaned them and then poured some red food dye coloured water over the screen and into the side wells. This red water came out by the front wheels with no problem. I have had the pollen filter changed during a service. (I was getting the water under the drivers seat with the old and the new filter!)
I have not tried the windscreen solution yet because I get the water on hot dry days only with the air conditioner working. I have also noticed the lack of water dripping from the underside of the car in hot weather as it used to do. (I would be grateful if any Sharan owner with A/C could have a look and let me know where the bulk of the A/C water comes from).
I am still working on the theory that there is a rubber valve behind the exhaust heat shield which may have stuck closed. I had a quick look under the car and there was a lot of oil / grease on the aluminium shield. I tried to wash the area but with no luck as the water under the seat returned today. I am unable at the moment to remove the heat shield. Perhaps someone could also tell me the location of this rubber valve and flap.
Thank you for your interest.
Colin
May 2009