Tenants have moved and left personal possessions in the house

Tenancy Types and Management

The tenants have moved and not handed in the keys, and left some of their personal possessions in the house, I arranged in kindness to pick them up and go for these and collect the keys off them but they were not in when I called. They refuse to hand back the keys until they get their stuff. We are letting agents and as they are paying no rent the landlady is going nuts and wants me to change the locks and store their stuff, can I legally do this, furniture is gone so they are not living its only personal items. There is only one key as they were in when she purchased and they have it.