I was intrigued to see the "Millbarrow" marked on a map in the papery TMA. Some searching revealed it had been a longbarrow north-east of Windmill Hill at SU094722, but was apparently destroyed by an uncaring farmer in 1863.
Aubrey drew it in his Monumenta Britannica. It was surrounded by stones - he called it 'an orthostatic peristalith' to be technical.
Stukeley also drew it in 1743, calling it 'a most magnificent sepulchre'.
The barrow was just west of Winterbourne Monkton, and if you go to that village's church you will see that the Reverend Brinsden's grave is marked by a sarsen from the barrow - supposedly the capstone of the barrow's chamber.
The site of the barrow was excavated in 1994, and the results are in volume 87 of the Wiltshire Archaeological and Natural History Society Magazine.