Have a look on Spareroom.co.uk - it's by far the biggest and best site for such things. You can filter for rooms that accept couples, rooms that are available for short term let's etc on there too. I've used it several times for both housemate and house hunting with success
Gumtree also has adverts, though the quality / level of crazy can vary more there
@Onesmallstepforaman You could look at doing theatre digs. Contact your local theatre to see if they have a digs list you could go on. Lots of touring companies want accom for 1/2/3 nights, 1 week etc.