Arundel Wildfowl and Wetlands Trust is just down the road from Arundel Castle in West Sussex. There is charge for entering the site if you are not a WWT member. On the day I visited it was £11.50 for an adult. The a large number of excellent hides and viewing areas, and the layout means you can get fairly close to the wildfowl there. A reasonably long lens 200-300mm would