Bit of history of the reasons behind PPP as it relates to NATS.
The Tory government pre 1997 had already earmarked NATS for sale, and had already budgeted for it's sale bringing in many million quids. Cue much hand wringing from the then Labour opposition, including the infamous "our air is not for sale" statement. (Sounds like "your pensions are safe", doesn't it? but I digress......)
1997 arrives and Gordon arrives at No.11, finding finances in a shambles. First thing he does though is ADOPT all the financial plans already in place, and puts a stop to any further borrowing. No tax and spend Chancellor will Gordo be, no siree. But there's a problem. £750m of a problem. Without the sale of NATS, which is against Labour policy, poor old Gordo is short of said £750m.
Solution, change policy like the wind. Use your majority in Parliament to force the sale of NATS through, as least a bit of it that rasies enough cash. Appease the masses with some claptrap about pension protection (you can always change this later when no-one's looking, say on Friday when something bad has happened in Iraq / Afganistan etc etc) and Robert's your Uncle.
Gordon fills the hole in the accounts, and also tidily sidesteps questions about CAA NATS future investment.
Governments can and do change policy, and legislation, at the drop of a hat when it suits them or their friends. The union did their very best to protect CAAPS both at the time of PPP, and after NATS enforced split from the CAA. The legislation is only as watertight as the Govenment will allow it to be. Do you trust the politicians?