How about [[Willy Howe]] and [[Duggleby Howe]]? they didn't have central burials. And there's [[Marden]] (but that's gone).
What can they be referring to in Dorset?
The implication seems to be Silbury, Marlboro, Marden - and Knowlton.
They are said to be isolated, not in groups, except Silbury/Marlborough. (Dubious statistical analysis I reckon.)
And not a word about Silbaby! ;)