I telephoned my local Audi dealer in Leicester and in Derby to try to find out if a cover was missing. Both their answers were the same, "Book the car in to our service deopartment and we will have a look". The VW dealer in Leicester was much more helpful, he emailed the page from the spares maual and from that I could see that there is no separate cover for the pollen filter. It did however show that the water deflector that fits to the bottom of the windscreen ( No 32 in drawing) is held in place by three clips (No 35 in drawing). The clips were missing from my car and there was a 2-3mm gap between in and the windscreen. I assume that the water deflector had worked loose over time because there was nothing to hold it down. Water was getting behind the water deflector and dripping straight onto the pollen filter. This problem had clearly existed for some time and made worse every time I washed the car with a hose pipe.
My thanks to all who have commented on my problem and to the Forum for inspiration to find the answer.
Water Deflector.jpg