This is a Scheduled Ancient Monument and is now recognised as the rampart of an Iron Age defended settlement, rather than a cross-ridge dyke as it was previously thought to be (and recorded as on the OS map). The ‘Magic’ website confirms that it was included in the schedule in 1970 as ‘Wiltshire 847’ and is now scheduled in the national monument register as ‘34190’, and is described as “Rampart of an Iron Age defended settlement 410m south west of Mount Scylla Farm”.
A two page report (PDF document) on the settlement is available via the ‘Magic’ website, at magic.gov.uk/rsm/34190.pdf
Iron Age defended settlements are actually very rare, and this is said to represent a well-preserved example. The site was mentioned by antiquarian John Aubrey who described it as a “rampard with graffe (ditch) eastwards, but no camp”. It runs from ST83177445 to ST83257435 and on to ST83237423.