Protected Tenancy issue - tenant not staying at property

I hope I can obtain some advice from anyone who might have had a similar issue?

I have a property which has a protected tenant living in it paying rent far below the current market value for the area. Her daughter, daughter's boyfriend and baby moved in with her a few months ago as they had been evicted from their own flat and couldn't afford anywhere else. I have suspected for a while that the tenant is not staying there most nights and have had this confirmed by neighbours who she has told she's staying at her partners most nights and allowing the daughter etc to remain in the house for a very low rent.

My question is how do I, or even can I, evict her for this?? In my eyes she is not living there permanently and the tenancy agreement is with her, not her daughter. I know I need to obtain proof that she is not living there but how do I do this? We are keeping a log of the nights her car is there and have spoken to the neighbours to whom she has admitted she's not living there permanently. All the rules seem to be geared towards the tenants rights - not the landlords! Any help would be much appreciated. Thanks