unpaid utility bills

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How many times should it be said- the tenant should be responsible for his own utilities - then you (landlord) are not liable in any way. Merely read the meters on entry, inform the utiliites of tenant name and let them get on with it. At end of tenancy read meters, inform utilities of tenants forwarding address and again, let them sort it out. oh, and advise them not to change supplier without contacting you first. Otherwise headache time will happen!!!
Other than that, I cannot really answer in honesty your original query - which wouldn't be a query if you follow above advice.
good luck!