Richard,
I searched the GRO index as well, and couldn't find him either. I thought it was just me, but reading that you couldn't find it either, just confirms my suspicions. Unless maybe Harry was born in Scotland, or elsewhere?
Unless someone finds evidence to the contrary, it appears the baron's younger brother, the Hon. Andrew Perceval (b. 1947) is hp. The only other male in the line of succession (on the DPB Online website) is the youngest brother, the Hon. Michael Elibank (b. 1953).
Brooke