Keeping entire deposit for damages to property

Tenancy Deposits

So for the first time in 22 years I've just had to retain the entire deposit after a tenancy has ended for damage to one of my properties. AST ended 17th Aug, they negotiated to stay on until 6th Sept. They didn't close any of their accounts with utilities either and kept a cat without my knowledge, the bedroom carpet was infested with flea's and cat pee. I am covered in flea bites so are my maintenance guy's. Tenants deposit is now unprotected with mydeposits.com. I have provided the tenants with a clear breakdown of my costs to repair the property. I have heard nothing from them so far. So where do I stand now? What can be done to end this misery for landlords?