Actually... again it doesn't... under the guidelines (aka Law) she could claim Child Maintenance from you and apply under the Child Support Act to stay living there until your child finishes full time education. Usually 18 years of age, however from April next year... technically... she could stay living there until your "Child" turns 20... just so you know
I know of successful claims where maintenance has been awarded to 22year olds to force their dads to pay for their University living costs... (just so you know that means to the mother, with whom the child lives with and therefore thats where those costs go).
When the CSA become involved... you have no chips on the table