Inform the letting agency, in writing, that you will stop paying the rent as from next month unless effective action is taken, repudiate responsibility for condition of flat, and demand a refund of any deposit paid and for rent for the period during which the problem has occurred. Best get a solicitor to write this. This will cover you against any claims from them - their only remedy is to go through the courts, and if you have the situation documented, the court is unlikely to take their side. Find the boy a new flat. No doubt his flat is already in use by dd and his mates.