Just to add to Alan's answer, if the farmer already has an agreement with C&RT then you need not apply to them for permission but you should specify the mooring when applying for a cruising license. If the farmer has no agreement, you need to apply to C&RT for a mooring license and the cost will be as Alan said. Such an application or renewal is not available on-line and has to be done by post or personal visit. Either way you can be pretty sure that one of the enforcement team, or whatever they are called nowadays, will eventually spot you wherever you moor and if you're not compliant, expect a letter.