Extra people staying over

Other

We let out a 1 bedroom flat and there is 1 person on the tenacy agreement. (6 months shorthold tenacy agreement 01/08/2012 - 01/02/2013)
She has had at least 2 people staying over sometimes already and we were assured that this was a one of incident but it has happened again recently and it has now come to our attention as the neighbours have complained to us re the noise and when my husband investigated he found her parents were in the flat.From what he can understand as they don't speak English they have come to england to visit her for 2 weeks. Does anyone have any knowledge on where we stand as the no of people in the flat far exceeds its size!