This hill is next to Croxton Abbey, whose Abbot looked after King John when he died in Newark in 1216. Apparently he got to take the king's entrails / his heart back (whilst the rest of the body went to Worcester). That's fairly gruesome an idea. You'd think the guts would have been put in the abbey, but the Remember Waltham On the Wolds website has the nice local twist that they ended up in this barrow (there's also a photo. Of the barrow, not the entrails.)
If you look at the scheduled monument information for the site, it says it's the remains of a medieval post mill. But it concedes it 'is thought likely to have utilised a well preserved Bronze Age burial mound.' Because the mound of a post mill wouldn't really be a suitable resting place for a kings innards would it. And the idea of reusing a prehistoric mound for burying people in later times is common enough.
(SK 820279) Windmill Hill. (1)
To the west of 'Old Wood', at Croxton Kerrial, is Windmill Hill, on which is a mound supposed to be a tumulus, which, however, has never been explored. (2)
A Windmill mound. Published survey 25" correct. (3)]