Having had no success finding the whereabouts of Horn Hay through the usual Google routes, I turned to the on-line catalogue of the National Archives. The property is listed as "Horn or Hoon Hay" in Derbyshire and searching for Hoon Hay on Google maps puts it indeed as a few miles East-South-East of Derby city.
The plot thickens though, as the deeds listed in the National Archive database gives the owner as Thomas Bradley Paget, the brother-in-law of Andrew FitzHenry senior (see the entries on the Fitzhenrys of Rock). Hoon Hay had originally belonged to the Rev. Richard Watkins, so it was not a FitzHenry property, but had passed to the Pagets by marriage.
After the long and bitter Chancery court case, had the families become reconciled? Was Andrew FH a resident in, or a tenant of, his uncle's property?