You're all missing the point.
The money left over was from changes to our agreements and was originally put onto arduous routes at both LHR & LGW. However with LGW going to SFLGW, all dest. payments are out of LHR only now. Its irrelevant if some routes were originally at LGW.
There has been more of these destinations recently and the 'pot' has been split more, hence the payment has dropped dramatically over recent years.
We're not getting anything extra - just monies that were owed to us from '97.