Rare

Hello everyone I'm new to this, so forgive me if I'm in the wrong place! I need advice. My family own a field by the river, and we've had people mooring up there for ever (well, all of my life and I'm 40, and dad can't remember a time they wasn't boats either) So, we don't charge much and we leave people to get on with whatever they want to get on with, live and let live and all that. We take good care of the land, keep the willows safe and the gate and drive good. We rent out mooring, not as a residential mooring, but like I say, we don't interfere with the boat people, some have made little allotments next to their boats, some have a little shed. Everything has been fine until this week. We get a letter saying we have had breeched planning permission and changed the use of the land? (It's been like this for 60 years at least) Saying we could face fines and wanting to know lots of info on what happening on the land. So, what's the law on this? How long can people stay on their boats a year? Can I make them residential moorings or is that an expensive can or worms? As a land owner do I have a say on what I want to happen? Any advice will be gratefully received, I want the people who have their boats to be able to continue to do as they wish with their boats. Rare x