My brother and his girlfriend have been living in a 'cabin' that was built by a friend of theirs (on said friend's land) approx. 20 years ago.
The friend lived in the structure (like a static caravan in size but wood and hand built) for around 12 years then my brother moved in when the friend bought a house elsewhere.
The friend still owns the land and is happy for my brother to live there. They have water, mains electric, a TV license and photographs to show how long the structure has been there but they never got planning permission and do not pay council tax.
Post gets delivered but all formal addresses (eg. bank, electoral roll) are my parents address. The site is quite isolated, it's on a farm track with only farms as neighbours.
What is the legal situation if the council found out this building was now being used for residential occupation? Would they automatically evict/ fine/ demand backdated council tax?