Area52 - there are numerous ways to lower the burden on NATS to keep the pension scheme as it is.
As you mentioned, making only part of a pay rise pensionable is one of those... unlike you, I don't think that's a horrendous idea (neither do all the Reps I know) - think how much pension a leaver gets today then add a little each year, it's still a good pension... it might actually be unfeasible to ask for a pay rise of RPI then expect all of it to be pensionable!!
The company can also take pension contributions out before paying us, thus saving employer PAYE contributions on that part of our salary. The end result is the same amount of money goes into the pot, just not double taxed; unfortunately NATS are not too happy to do this as they know it makes the pension extremely viable.
A smaller increase in pensionable salary with an extra bit non pensionable is not too bad - what we do not want is a closure of the scheme as that could cause problems later on. Anything we can do to keep the scheme as it is needs to be done.
However, the unions do need to be strong, as the pension at the moment is in a very healthy state... it can be bolstered by having NATS pay the correct full contribution.
As for customers paying for our pension - that's bull. It's a very weak argument by NATS. Every company pension scheme is paid for by that companies customer... it's one of the differences between Gross and Nett profit FFS!