There are three ways the DB scheme can be changed. Every single member agrees to the change, an Act of Parliament reverses the protection in the original Transport Act, or NATS goes bust and enters Transport Administration. The first is not going to happen. The second might, for the first time since PPP there is a government with both the inclination and the necessary majority to do it, but it would depend on them finding parliamentary time and they have other pet projects to favour. Even if they change the legislation it is possible that the agreement NATS signed to get previous concessions might still stop them closing the scheme. The third is likely unless either NATS cuts its costs to match its income or finds a generous benefactor willing to stump up the cash.
In the last Annual Report 1990 employees were members of the DB scheme against an average of 4464 employees in total. Those figures are a year old and a new Report is due soon.
NATS would dearly love to close the DB Scheme. It's size relative to the size of NATS means that if the scheme falters because of factors completely outside of NATS control the NATS balance sheet takes a big hit and last year its contribution to each DB Members pension was >40% of pensionable salary. The fact that it hasn't been able to do so is testament to the provision of the protections in the Act for which members can thank the late Lord Brett previously the General Secretary of IPMS, now Prospect.
Public ownership may be the only way forward but if you are an employee be careful what you wish for. Take a look at how public sector worker salaries have fared since 2008 and compare them with those in NATS. It is very likely that public sector workers will be those footing a lot of the bill for the money the government has recently been throwing around.