just one of the procurement rules is that all projects should have a Post Project Evaluation (PPE) conducted
Precisely. It is (allegedly) the way MoD learns from experience. The rule also says you prepare the PPE around 2 years after ISD, by which time anyone who knows anything about the project is long gone. Not once did I get any feedback about a PPE.
Pontius. As Phil says, there are many answers, one of which is the Defence Scientific Advisory Council, whose reports direct to Secy of State are usually secret; except when quietly handed to favoured contractors to "help" them with bids. People look at it different ways. At least your example is only obsolescent, not obsolete. The latter is common, so be thankful for the former!