Crag Gill is a house on the western side of The Lake.
Miss Thornton lives there; she is the only contemporary of the Great Aunt left, but “she (the GA) had a row with her ages ago. They haven’t been on speaking terms for years”. So there is no danger of her giving the Ds away to the GA, as Nancy tells the Doctor (PM8). Later the GA leaves the butcher’s van, rather than drive up to Crag Gill in the front seat of the van, to which he is delivering a leg o’ mutton (PM23).