Perhaps the idea that the Marlborough Mound is not merely a motte is not a new one. In 1782, in his ‘Observations on the River Wye’, William Gilpin (the prebendary of Salisbury) wrote:
Marlborough-down is one of those vast, dreary scenes, which our ancestors, in the dignity of a state of nature, chose as the repositories of their dead. Every where we see the tumuli, which were raised over their ashes; among which the largest is Silbury-hill.. At the great inn at Marlborough formerly a mansion of the Somerset-family, one of these tumuli stands in the garden, and is whimsically cut into a spiral walk; which, ascending imperceptibly, is lengthened into half a mile. The conceit at least gives an idea of the bulk of these massy fabrics.
Could this have been an idea he was told as a visitor – that the mound was a giant barrow? Maybe he heard it was Merlin’s burial mound.
Quoted in ‘Notes on Marlborough Castle’ by JHP Pafford, in WAM v60 (1965).
I don't know much about this but I believe there is supposed to be a similar mound in Lewes, East Sussex (well Ive climbed on top of it the mound exists ok) which must be closeto marlborough mound in size, and is likewise believed to be similar to silbury hill, although I know of no archeology to support this.
Interesting.