Hard post to follow. Not quite with you Kim, see below.
What mistakes? anything relevent to the contract?
Standing order? do you mean direct debit? i'm sure Standing orders a fixed payment you set up and pay each month from your end.
What do you mean there?
Did this happen or is this the idea?
I think regardless of the contract, they null and voided it if the idea happened and some other girl moved in under your contract term.
They owe you the money if what i gathered from your post is correct.
They could have also done something unlawful by moving someone into the property you are renting without your knowledge. Sue!