The Trust of Promise is a very strange legal device which wasn't understood at the time of PPP and isn't still. I've yet to find anybody who will tell me exactly what it means and if union reps are briefing that it only comes into effect if NATS is badly managed I don't think they know either.
There were two protections at PPP, the protection to the Trust Deed contained in the Transport Act and the Trust of Promise which the Government required all the prospective bidders to agree to before buying their stake in NATS. The first protection stops the pension benefits being changed and the Trust of Promise stops you being forced out of the scheme or the scheme being closed.
Quite how it does this is a mystery and we need to find out exactly how. As it's an agreement between the Government and the buyers it can't be something that only comes into effect if NATS fails. More likely it somehow prevents a sale by the original buyer without a similar agreement to protect the rights of pre 2001 members. It is quite likely that this will lapse if the company goes bust but that needs to be confirmed.