I suppose there is some kind of informal 'market' between airlines with differential pricing for the 0900ish and 1700ish slots versus the rest? Does NATS get the lion's share of the premium paid? Because perhaps NATS should, and perhaps NATS should have set that premium. Or maybe they do already and the demand is still overwhelming? You'll perhaps forgive me if I have a feeling the simple matter is that NATS do not have a say because BA call most of the shots at Heathrow on a legacy basis?
Unfortunately neither NATS nor the BAA receive any kind of financial benefit from the allocation of runway slots at Heathrow, nor indeed are they involved in any way shape or form in their allocation. This is carried out by an independent company called Airport Coordination Ltd who administer runway slots for all the UK co-ordinated airports. Allocation of slots happens at inrernational conferences ahead of each season. Certainly airlines buy and sell slots amongst themselves but the airport and air traffic providers are at 'arm's length' from the slot process. Grandfather rights is the proper term for your phrase 'legacy basis. These are tightly controlled on the basis of 'use it or lose it'.
Now, some might agree with you that these slots OUGHT to be auctioned off to the highest bidder, with BAA & NATS recouping the gains to pay for service provision, an interesting debate.
TheOddOne