If Liverpool and easyJet have entered into such a contract, I would imagine both parties would be fervently hoping that it will not get anywhere near a court of law. I suspect that certainly Liverpool would be held as having acted in contravention of competition law, in that their actions could have jeopardised the choice of air routes available to consumers in the north-west region.
There would be a number of issues to investigate, not least the interaction between MAN and LPL airports to serve catchment areas which are distinct and which overlap. However, the court case would be hilarious. From a position of little knowledge of this, any action by Liverpool to try to enforce the terms of such an agreement (if it exists) would be laughed out of court as having been illegal in the first instance.
Should be a good laugh if it gets that far though. One wonders whether easyJet's not-insignificant slot application for Manchester in Summer 2008 is a means of testing Liverpool's resilience and willingness to enforce whatever agreement might be in place.