Unfortunately, I think you missed a golden opportunity to serve this person notice of eviction of their possessions when the house ownership was transferred to you. You do need to consult a solicitor and get them to serve notice now, without any further delay. When a very dear friend of mine was selling her flat and her shop underneath, she asked if she could put the surplus stock from the shop, a hardware business, in our garage. My husband refused as we really did not have any spare room, but he did find her a garage to rent near her new home. She filled that garage with the stock, and her own double garage and three garden sheds! That was over 16 years ago and she has never sold one single item, it is all just rotting away! The rent she has paid all that time far exceeds the value of the contents!! We would really have been lumbered if we had agreed!