Well I beg t differ but …
The agents may pay a concession to the airport (some do some don't) and this may be included in the airports aeronautical revenue but this is not indicative of what the airline pays.
Eg is airline x is paying £450 a turn for an A319, the agent may pay the airport 10% at most, more likely less and this maybe included in the charges listed as the airport, but these reports will not include the full £450 as this will show in the handling agents accounts and is not split by airline/airport as a rule.
I stand by what I said, that SEN is a particular case as it is unusual for an airport to do everything.
But even if this isn't completely true, anyone who maintains SEN are charging airlines more than any other airport in London is seriously deluded. easyJet simply would not be there if that was the case ...