tenants 'guests'

Other

We rent out a 2 bedroom flat to 2 persons who are friends. We have discovered that one girl has her mother staying with her in the same room who has come from China. Before renting out the flat we had to sign a form giving the flats address as proof that she had an address in England . We assumed that this was just a formality and the mother would be living elsewhere.
We used your tenancy agreement and the mother is not named as being allowed to live there.
We obviously have to find out how long the mother is intending to stay. Is she just visiting or is she living there that is the question.
The girl with the mother has not had the courtesy of asking the main tenant if it is ok for her mother to stay and it is an invasion of his privacy plus as the mother is in the flat all day the electricity bills will be higher.

Any advice on this matter ?