The RN website says that Rowner and Grange are both within the perimeter of HMS Sultan, so they would both appear to be closer than the clues require:
http://www.royalnavy....ts/HMS-Sultan/History
However, Grange does seem to fit the "two moats" of the fifth fort's description:
http://www.pastscape....ob.aspx?hob_id=234447
Given that Rowner is barely half a mile north of Grange I reckon Fort Brockhurst is closer to being half a mile north of HMS Sultan, and a total of one mile away from Fort Grange.