Interesting. I'm not going to comment on the internal or metaphysical aspects, but the inspiration side is looking just as mysterious.
It is, I understand, established historical fact that the Great Barrow "is"
Wayland's Smithy, about 30 miles from Stonehenge as the craban flies. I've driven there from the south, and you really do drive across the Barrow-Downs to get there. It's a fantastic monument on the landscape, but
not associated with a hollow containing a single standing stone.
The obvious candidate in this part of the world would be a hill fort, and indeed,
Uffington Castle is very close by. The famous chalk white horse doesn't directly appear in Middle-earth, though Rohan and the Prancing Pony both reflect it, and I understand Dragon Hill has been strongly linked to Weathertop. But... Uffington Castle is bigger and more complex than the "hollow circle" on the Downs, and doesn't have a standing stone.
Another key location nearby would be
Lambourn Seven Barrows, within about 2 miles of Wayland's Smithy. These give a good picture of what a typical barrow on the Downs would look like, and I'm seeing descriptions of at least two of them as "shield" or "bowl" barrows - ie, you go up the mound and down into a hollow. There's no standing stones, though.
And neither of these options really makes me happy. You don't
put a standing stone in the middle of a barrow, because that's where you put the dead people! And hill forts are normally much bigger than Tolkien's description sounds - you can fit a village inside them, so a single stone wouldn't really stand out, and you don't feel like you're in a hollow.
What we really want is a henge. Not a
stone henge (Stonehenge itself is way off), but a henge in the original sense: a circular earth enclosure, smaller than a hill fort but larger than a bowl barrow - and, for our purposes, with a single standing stone in the centre. Something like
Mayburgh Henge is a very good match, except for the minor fact that it's over 200 miles north of Wayland's Smithy and the real Barrow-Downs. I can't find anything close by.
So... slightly to my disappointment, despite "we fell asleep by the stone and then the kids got lost in the barrow" sounding
exactly like the sort of family story Tolkien would have put in the book (see: every part of
The Hobbit in the vicinity of the Misty Mountains), it looks like he might actually have made this one up. ^_^
(Though there
is a survey marker on the east bank of Uffington Castle. It's not a traditional standing stone, but it's an impressive concrete monolith nonetheless...
here's how it looks from inside the hill fort. Not perfect, but very close, and in exactly the right place; maybe my first idea was right all along.)
hS