Housesitter issue

Tenancy Types and Management

We have a house sitter who is supposed to look after the house for us when we are not in the country. We work aboard but we come back to this house. We issued her an AST by accident, stupid me! AST said rent is 0.

This house sitter has been there for 2yr and she is not doing her job well. E.g. not posting us letters every week, as a result we missed payment; got drunk and lost the keys, called someone random to break the lock; freaked out when she could not stop the wired alarm system and called someone in to break the whole system so it cannot be used again; not taking meter reading every month.

Does she have tenancy rights? We gave her 1 month notice to leave but she wanted 6 month. She is not returning the keys and leaving her possessions there. Can we change the lock? What can we do with her stuff legally?