Another story about the Bulford stone in the river:
In the bed of the river, just above Bulford, a great Sarsen stone lies, like those of which Stonehenge is built; how it came there is not known. The traditional explanation is that the Devil, having, by Merlin's command, bought the stones from an old wife in Ireland, bound them in a withy and flew hither with them, and as he was crossing the river at this point the withy slackened and one of the stones dropped out. There is another stone of the same kind in an upland field to the west.
I suppose this isn't the stone Baza's taken a picture of? But maybe another Bulford stone to search out I suppose.
.. a stone formerly in the Avon near Bulford, in the bend just south-west of Watergate Farm (SU 16054330 [this is at the foot of a slope crowned by a long barrow]). This is less than a metre across, and its upper side has been cut to form a square socket, and to the slab is fixed an iron ring possibly for mooring a boat. It is in fact a slab of oolitic limestone and has nothing to do with Stonehenge, yet it has already gathered 'megalithic' folklore.
A farmer with his team of oxen is said to have tried without success to move it from the river (Long 1876, 75, note 2). 'An observer' wrote in the Salisbury Times (11 March 1910) that 'several attempts have been made to drag the stone from the river. Forty, and some say sixty oxen were employed, but it was never even moved.' Similar traditions occur at the Rollright Stones and many other megalithic sites in Britain.
The stone was mentioned in the poem by F. Bowman (1823), 5: No kindred relics boasts the neighbouring soil,
Save one rude rock, that rests its time-worn side
On Avon's bed, and curbs his struggling tide.
Scarcely compatible with the supposed immovability of this stone is another local belief that 'whenever it is turned over it always rights itself again' (Emslie 1915, 167).
When this stone was removed from the river ten or twelve years ago, the men allotted to the task were at first reluctant to have anything to do with it, believing that there was a curse on the stone. The latter is now in the garden of a house by Bulford Bridge (inf. Dr Isobel Smith and the Wessex Water Authority, Poole).
From The Legendary History and Folklore of Stonehenge
L. V. Grinsell
Folklore, Vol. 87, No. 1. (1976), pp. 5-20. The eminent Grinsell also reminds us in this article of another water bound stone at Figheldean, which Aubrey says was supposedly dropped on the way to Stonehenge* (Figheldean is a little further north, so curiously not really on the way at all).
Long's work is 'Stonehenge and its barrows', and Emslie's, 'Scraps of Folklore' in Folklore 26. Emslie has this to say:
Bulford Water Stone, near Amesbury, is a stone in the middle of the River Avon. On its north side is an iron ring, fixed in it, and which always lies upon it in a direction which is opposed to the current of the river. It has frequently been turned over so as to lie in the same direction as the current of the river, but has always returned to its original position by going against the current of the river. [collected 1896]
Here's something a little earlier which I found in 'The Beauties of Wiltshire' by John Britton (1801):
About two miles north of Amesbury, on the banks of the Avon, is Bulford. Near this village are two large stones of the same kind as those at Stonehenge. One of them is situated in the middle of the river, and, as I am informed, has an iron ring fixed in it; but the waters being very high I could not see it.
The other is on the Downs, a little to the south-east of the village; and about a mile further up the valley is another, all evidently appertaining to the structure I have already described [i.e. Stonehenge]; but whether they were ever brought from the circle, or were left here on their passage, on whether they belonged to an avenue stationed between Stonehenge and Avebury, it is impossible to determine.
And a little more, on the stone that was in the water (this is from p229 of v1 of 'Miscellaneous Tracts' by William Withering - 1822). It's from a letter by James Norris to Dr Withering, dated Feb9, 1798.
I was at Bulford again in August last, and conversed with the farmer who occupies the estate on which it lies; he assured me he had been upon it when a long drought had laid dry its surface, and that the ring is certainly of iron. But I found him inclined to invalidate the opinion of its antiquity, by relating a tradition, which I will here repeat: 'it is said, that, formerly, a railing extended across the river at this place, to detain the fish: that the square cavitiy in the stone received one of the supporting posts: that another similar stone was once placed in the same river also, near the opposite bank, for the same use; and that the ring is of later date, and fixed only to attempt the removal of the block.' Be this relation true or false, I cannot but think it improbable, at least, that so much needless trouble and expense should be incurred, when a post firmly fixed in the earth of each bank only, would have been fully adequate to the purpose. He says the nature of the stone is different from any of the three kinds at Stonehenge: that it is softer, and agrees with the productions of the Chilmark quarries, situated about fifteen miles south-west of Stonehenge and about twelve miles west of Sarum.
*From Bulford I went to Fighelden, and made many particular inquiries of aged and intelligent natives of the place concerning the stone said by Aubrey to lie in the river there. Their invariable reply was, ' that none such was ever known to exist at Fighelden, or nearer than Bulford; where (added they) is to be seen one corresponding with the description.' It is alsmost certain, then, I think, that the Bulford stone is the real object of that writer, who has fallen into a local error in the name, and in about three miles in the situation of the place.