On some cars this can be associated with a clogged air conditioning drain. Do you have a sunroof? Looks like you are from the U.K. so wouldn't figure you would have ac on this time of year. I heard cars from your side of the pond tend to have rust, how is your floor pan, frame rails and weather stripping?
"If you ain't moving fast enough your costing me money and if your costing me money you ain't moving fast enough." -"The Machine"-