Jon,
www.homefarmradnage.co.uk/facilities.html
You don't actually say where you are travelling from but we stayed at Home Farn C&C Park last year when we went to Wembley to watch Warrington in the Challenge Cup Final. The site is located at Radnage on the outskirts of High Wycombe.
Decent site for a short visit and was ideal for us as we travelled from the North West along the M40. Quick taxi ride in the morning to High Wycombe station and a train ride on the Chiltern Line direct to Wembley.
I am sure that there may well be closer sites but the site certainly worked for us, especially as it is located next door to a pub!!! so we could have a celebratory drink when we got back that evening.