An obscure site, but very much worth the effort as this is a classic long barrow - combine with a trip to White Sheet Hill or/and Cley Hill if you aren't local... [update - more to the point the excellent long barrow and round barrows on and around nearby Cold Kitchen Hill]
In Monkton Deverill, take the minor road past the church and keep left where it does a sharp left-hander. Ignore two footpaths on the corner to park further on near two more public footpaths on the left (before you reach the track to Pertwood Farm). Head for the prominent copse of trees to the north and skirt this on its left. The long barrow is visible beyond, easy access.
Very well preserved, including ditches, Dyer gives it at 2m high and 76m long..... although it felt taller to me - perhaps it was the ditches? An usual feature is a berm separating the ditches from the mound itself.
Atmosphere in bucketloads, too, since you won't get any tourists here. Simply a great afternoon's hang.
Grinsell (quoted in Dyer's Southern England guide) says that this long barrow is unusual in that it has a wide 5m berm between the mound and the 'well preserved' side ditches. It's thought to be unexcavated. The map shows it lying parallel to the slope on its Wiltshire chalk downland, so I guess it presents the largest and most prominent view when seen from the valley below.