I've had two 12ft x 12ft stables on my land for 2 years without moving them and the Council havent said anything?
Some councils are great, some are ruthless
Think main problems arise from neighbors or busy body on the local parish councils
If i was you i would check to see if i have any photos from when they were first put up.
Take a couple of photos every year after 4 years they will be immune from enforcement anyway and classed as a permanent building.
Be careful if you add any more as it may draw someones attention to them and bear in mind you can repair them but if you replace them they loose the immunity and will need planning permission