If this is true then are NATS management or the trustees not guilty of mismanaging our pension fund?
Not at all. The obligation on NATS is to fund the scheme at 100% which they have done even when many other schemes were running up huge deficits. The underlying rate is that needed to fund the liabilities in the absence of any surplus, the actual rate is agreed between the trustees and NATS and in times of surplus can be lower and in times of deficit may need to be higher. In 2003 as the valuation report states it was possible for NATS to pay less for about 6 years and that is what has happened. Any contribution prior to that is irrelevant as the fund was in surplus indicating that NATS was providing more than enough.
There has never been any requirement to fund beyond 100% in actual fact taxation legislation meant that storing up surpluses was discouraged until 2004.
The main driver for the significant rise in underlying rate is the guidance from the Pensions Regulator that Trustees should assume much longer life and generally increasing life span. Each year of additional pension adds a great deal to the liability.
If they have not followed what the reports said then it is up to them to put things right
Even if that were true, and I've pointed out above why it isn't, exactly how do you think that could happen ? There is no big pot of money secreted away. This is all happening because NATS has signalled that it probably can't afford to fund the scheme if the underlying rate is 40% and if it can't pay your pension is under threat. You can argue about whose fault it is but personally I'd rather do everything I can to do something about it.