Paxboy
There is, and as far as I know, has never been such a thing as a 'fixed slot'. Eurocontrol do not give favour for higher fees.
Very few airports in Europe have landing slots. LHR does, FFM and maybe a couple of others but 99% of the time the landing slot does not have any effect on the dearture time.
The slots normally come about due to the high intensity of traffic through a certain region/route which may or may not be part of an arrival/departure route to or from an airport.
Sometimes, and only sometimes, it's possible to resolve the allocated departure delay by re-filing a new flight plan using a different route (assuming you have the info as to where the hold up was on the original plan) or the same route at a different flight level.
The bottom line is, when traffic density is high, everyone (whether BA or Honest John's one plane wonder) can suffer the pain and misery of slot delays and there's s@d all anyone can do about it!