Hi Chazdaz - just a few things for you to consider:
-are you in Scotland or Wales (as legislation differs for each)?
-Would someone have had to have been on your property to view the structure? perhaps trespassing to do so?
-do you have any old plans of the previous buildings (you will still need to apply for permission, but it might help your case for a temporary structure if you can prove there was something there originally)
-have you fallen out with any neighbouring land owners when shipping your cabin parts in i.e. tractor ruts/ tree damage from lorries/ vans - this might give you an idea of who has reported you.
- If it is one of your neighbours - start being very nice to them. Like wise with the planning officer - whatever you do, don't argue - they are entirely partial to being more lenient with a polite but knowledgeable person - lets face it, they get shouted at enough! so it makes a change when they meet a patient land owner. I don't mean bake them a cake - be personable - invite them onto your property for a look round and a chat and show them the plans from the previous dwellings/ outbuildings - Ask for their expertise & advice - Flatter them.
Take a look at this link which you might find helpful:
http://www.woodlands.co.uk/owning-a-wood/woodlands-and-planning-legislation/
It also suggests that you will need planning - but it is rare to get a dwelling application passed - hence you will need to emphasise 'Temporary', 'Moveable', and 'Previously there was such and such..' in your application.
Failing that - find some pipistrelle bats quick smart and ask them nicely to hang off your rafters whilst the Buildings Officer is there!
Your best bet is to call your local planning department and ask for an on-site meeting with the officer to offer them your proposed plan of action/ solution (be positive) and go from there.
Good luck!