NATS does not currently benefit from Govt support for its pension plan.
Most of the actual pensioners in the pre-privatisation phase were put in the CAA's part of the plan.
The real cost was in the privatisation process, when HMG stuffed the CAA and the NATS pension funds full of taxpayers money. That was because prior to the privatisation, the liabilities had been HMGs.
However, if either the NATS or the CAA's pension plans hit problems, they will rush to HMG for more money. It's one of those "Is the Pope a Catholic" sort of questions.