The Letting Centre

My tenants informed me by garbled text message on 23rd August that they were splitting up and leaving my house on 9th September and that they would not be paying any more rent. Their tenancy normally runs to the 29th of the month.
I attended the property today (15.09.07) to find that they have vacated the house leaving it extremely dirty, all upstairs windows wide open, a pile of faeces in one of the bedrooms, the garage and rear gate wide open and a considerable amount of their belongings still in the garage, along with some clothing and shelf units in the house.
I contacted them by phone and text over seven times throughout the day but they failed to respond.
So I took moves to secure the house and I changed the locks. Am I legally ok to have done this on the grounds that they'd breached their tenancy through non-payment of rent, they have already vacated and my concerns for the security of the property?
With regard to their belongings, I have asked them to contact me to arrange a convenient time for their collection. How long do I have to give them to respond before I am entitled to clear their possessions out and what should I do with them as I have no forwarding address?
I know this is a bit like bolting the stable door after the horse has gone but I would be grateful for comments.