Tenant vacated property during contracted agreement

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Hi

ive had a very difficult tenant that has sub-letted, been in significant arrears and been  allowed a puppy to live at the property to name but a few transgressions. Amongst all this, when things were looking better I stupidly sent her a new agreement. She and her sub-letters have now vacated the property but she's not left the keys or told me the flat is empty. I'm concerned about safety as there are other flats in the building. Can I change the locks even though she has a tenancy in place if I let her know the locks are changing?

Also, I was suspicious about her sub-letting but didn't know for sure as she or the people she'd moved in wouldn't let me have access. Now, the sub-letter says I owe him £350 as he was forced to move out by the tenant in the middle of his month there. I've never received a penny from him and have no agreement with him. Where do I stand?