Student Tenants have extra people living with them

Tenancy Types and Management

I have a 4 bedroom student property with 4 friends on one lease as from 01/09/13. I suspect that they may have 2 additional people living there.

The extra people will still not make it licenceable as the property is only on 2 floors, but are there any other implications that anyone can think of?

My first thought is extra wear and tear on the property and the cost of utilities. As is usual for student properties in this area, I pay bills up to a certain amount and they are required to pay the remainder.

Our lease says that they must let us know if other people are living there and "we will not refuse permission unreasonably".

Is all that's required a gentle email saying we suspect more people are living there and reminding them that they are responsible for any damage and any bills over and above the agreed amount?

Any advice would be much appreciated as I'm concerned not to miss any more serious implications.

Thanks v much