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My best guess is that this is where the Cowgrove Road turns off the bottom of St Margaret's Hill, the large building in the picture being the St Margaret's Almshouses.
Searching the census records turns up a family of Dacombes living at Chain Gate in 1881, and although no street address is given, it is enumerated in the same cluster as Farr's. Twenty years later, a single George Dacombe is listed as living in St Margaret's Almshouse.
Back in 1881, the head of the household, Samuel Dacombe, is listed as a Toll Gate Keeper. Whether the toll was into/out of Cowgrove, or rather on the Blandford road, I'm not sure, although in the photograph there is a large white gate on the Cowgrove Road turning, with a small window looking out on to it.
If this picture is from the 1880s, there is every chance that the Dacombe family, or members of it, are in the picture.