Well , you may have been fobbed off by the council , but then again you may not.
I know moorings are collected by the EA for Barge Walk , but i suspect the slipway is a local authority slipway ( don't hold me to that though )
Best thing you can do is to give Steve at Molesey Lock a ring and have a chat with him , he will be more clued up on who manages / owns what at that point.