If I'm looking at the right things...
The final document is available here:
Planning Policy Statement 3: Housing
I think the relevant part of this final version, which makes no mention of hospitals either, is this (from Annex B: Definitions):
Previously-developed land (often referred to as brownfield land)
‘Previously-developed land is that which is or was occupied by a permanent structure, including the curtilage of the developed land and any associated fixed surface infrastructure.’
The definition includes defence buildings, but excludes:
There is no presumption that land that is previously-developed is necessarily suitable for housing development nor that the whole of the curtilage should be developed.
The previous document is available here:
Planning Policy Guidance 3: Housing (Cancelled) - Annex C: Definitions
(oops - looks like rotorcraig posted whilst I was still typing!)