Listed, in no order of merit:-
a) Caegynon - We used this for 3 or 4 Easters around 1980. On arrival you chased the sheep out, mucked the building out, and then moved in. At that time this part of Wales was dry on a Sunday, but the landlord of the Druids Arms (being a Brummie) would allow English in (but no Welshmen 'they might start singing hymns'!).
b) The Chicken Sheds at Spartylea, near Allendales. A collection of fairly ramshackle buildings with a chemical toilet shed which bore the notice 'When full take to centre of bridge (over the stream) and empty into centre of stream'.
c) Winterings - the hut of the Moldywarps in Swaledale. This was as near a derelict buildings as I have ever paid money to stay in. It was also bitterly cold when we used it, and you could only get anywhere near it with a Landrover.