Anotherthing
I will try to explain this but I would urge you to go to a briefing because the actuary at ours explained it far better and more accurately than I can and, as somebody else pointed out, she was also much cuter than me.
The pension scheme has current liabilities based on the benefits people have already amassed and the current pension pot should be big enough to fund those liabilities. The scheme is in surplus or deficit depending upon whether or not it is predicted to meet the current liabilities.
The scheme also has predicted future liabilities caused by the further benefits its members are expected to earn before they retire. The underlying rate is the contribution rate required to ensure that these added benefits are sufficiently funded as they are amassed.
The underlying rate depends upon those future liabilities and doesn't change with surplus or deficit. However if there is a surplus NATS can pay less tha the underlying rate until the surplus is reduced to the level agreed between NATS and the Trustees.
Changes to assumptions have an effect on both current and future liabilities but, because of the timescales involved, are likely to have a far bigger effect on the future liabilities thus driving the underlying rate.
A deficit is addressed in a different way. That requires a restoration plan whereby NATS will pay the deficit over an agreed period of time. Ten years seems to be figure regarded as normal. The deficit is not addressed by increasing the underlying rate.