I seem to recal the fee payable for a longer stay is to do with rockland staithe and nothing to do with church fen, I believe there is also a similar fee to use the slip at rockland.
Not sure why everyone is getting hot under the collar about it, any mooring should be used as "at your own risk" and decisions made as you approach as to wether or not it's safe to moor there, that is basic navigation skills as well as darwinism, most moorings have the ability to be slippery as hell when wet.
Disclaimer: I don't care if anyone overstays at the non BA part of church fen as it's not my business, I also don't care you slip over doing so as I won't feel it, however if you catch it on video I would like to see it as it could be funny!