The size of the deficit is directly related to the contribution break and reduced contribution levels!
Given that the scheme was fully funded prior to the recent hiatus in the stock market the size of the deficit, if we assume there is one, is related to the funds exposure to shares and the recent fall in value of those shares.
Any deficit will be tackled by an improvement plan agreed between the Trustees and NATS and not the underlying rate. Provided that shares recover it will add to the problem but only short term. The underlying rate is driven by the other factors and it is that which concerns NATS and that which the proposal addresses.