As I understand it, the Pension Trustees can only force NATS to make up the shortfall in 100% funding - i.e if the fund is in deficit- and thats where the "recovery plan"* comes in. The deficit occurs when values of assets go down (as they have in recent financial turmoil) and when contributions are low (just like what NATS have been doing). The trustees will not demand 37% for underlying costs but enough for the fund to be able to contnue paying out for current pensioners (and not future pensioners)
*recovery plan: a document summarising a plan of action for correcting a shortfall over an agreed period - source: 2006 Triennial Valuation