Iley Oak was said to be the place where King Alfred and his troops rested overnight before the battle of Ethandun. The oak was later a favourite (and less legendary) meeting place of the non-conformists of Crockerton, who held their religious meetings in secret there, at the earthwork called Robin Hood's Bower.
Iley does look like Ilegh, which was the meeting place for the hundreds of Warminster and Heytesbury until at least 1652, according to the Victoria County history (see http://www.british-history.ac.uk/report.asp?compid=16071) - and this was described as the site of a great tree standing in Southleigh or Eastleigh woods. Southleigh is where RHB is. Ah, it all comes together you see.
Completely bizarrely, according to discussion on the Megalithic Portal, RHB is planted with monkey puzzle trees, and you can also see (encouragingly in light of the above) the remains of a stump of a large deciduous tree (an oak? which would fit in with the story - and make a link to Robin Hood and his oaks in Sherwood Forest).
But what is Robin Hood doing so far south? It's all terribly confusing.
The record on magic doesn't have much to say about the site:
The monument includes Robin Hood's Bower, a sub-rectangular prehistoric earthwork enclosure on low lying Greensand south of Warminster.
The monument comprises a sub-rectangular area of 200 sq m enclosed by a ditch up to 1m deep and 7.2m wide and a slight inner bank 3.3m wide and up to 0.2m high. The enclosure is crossed by a modern track.
SOUTHLEY-WOOD, so called from the circumstance of its lying to the south of Warminster, is distinguished by a small intrenchment, denominated Robin Hood's Bower, which is nearly of a square form, and comprises within its area about three-quarters of an acre of land. Close to the eastern boundary of this wood is another similar earthen work; and on its eastern side is a third intrenchment, resembling an amphitheatre in miniature. This is a very curious work, and consists of a ditch and two valla. The outer vallum is about eighteen feet in height, and is very neatly formed. -- "The breadth of the ditch is seven feet; the height of the inner work from fifteen to sixteen feet; and the length of the area of the inner work on its longest side (for it is of an oval shape) is one hundred and eleven feet."
I don't know what they're going on about, do these other places still exist or has someone got the wrong end of the stick? The quote comes from Hoare's Ancient Wiltshire (v1 p50, apparently). The second could be the henge at Ivor Farm, but the third sounds so large as to be unmistakeable and unoverlookable. Hmm. http://books.google.co.uk/books?id=pi1JAAAAMAAJ&pg=PA307
From 'The Beauties of England and Wales' volume 15, by John Britton and others (1814).