Thank you for your question and welcome. Is the tenant's name on this unpaid electricity bill? Have you been sent a copy of the bill that is unpaid? Kind regards AJ
To answer your question, as long as the bill is not in your name you are not liable for it. To become liable for another persons debt you have to guarantee them and this has to be done in writing, this is one of the oldest legal principles in the Statute of Frauds 1677.
I would write to the utility company and say they have not provided you with evidence of the debt or why your company (as landlord) is liable, say it is a criminal offence to harass someone for money that does not owe it under S.40 if the Administration of Justice Act 1970.
Say that if they do not stop chasing you, you reserve the right to report the matter to OfGem.
The only issue is, if the invoice and account are in your name then you will be liable for it. Kind regards AJ