Somerset has 11,500 listed buildings, 523 scheduled monuments, 192 conservation areas, 41 parks and gardens including those at Barrington Court, Holnicote Estate, Prior Park Landscape Garden and Tintinhull Garden, 36 English Heritage sites and 19 National Trust sites, including Clevedon Court, Fyne Court, Montacute House and Tyntesfield as well as Stembridge Tower Mill, the last remaining thatched windmill in England. What is the most valuable National Trust site in Somerset