Tenant changed lock without permission

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I have a tenant who has moved out of my property without notifying me but he changed the locks without telling me until I notified him today (06/12/2013) that I would be going to inspect the house on Monday (09/12/2013) after which he told me. He believes that I have overcharged him whereas, to the end of his notice period (he told me that he's sent a letter on 21st November but I never received it) I calculate that, if the Housing Benefit office are as good as there word, he will owe me £185.80; if they don't make the final payment that they have said the amount rises to £611.73. He refuses to give me a key to the lock he had installed until I pay him an amount of money which he believes that I owe him but is not prepared to prove. Can I have a locksmith get me in and then change the locks so he can't get in without me giving him a key?