The company is certainly within its rights to take a payment holiday when there is a surplus, as now. The Trustees are not legally able to prevent it (as I understand it). And of course there is no risk to the staff, due to the Trust of Promise preventing NATS from replacing the scheme with an inferior one.
But how about this for a dark thought? Due to recent stock market falls, the surplus has shrunk from £500M to £300M. If NATS takes a payment holiday and the market continues to fall, the fund may become unviable - its assets too small to meet its projected liabilities. Then the Trustees would be obliged to close the fund. The Trust of Promise would no longer apply, and NATS could set up a cheaper, and inferior scheme.
Of course I am sure that is not what management would want. Is it?