Reference: [Add details]
On [dd/mm/yy] I received a [bill/ demand for payment/ confirmation of an order placed] relating to a contract that I have no knowledge of.
I have enclosed a copy for ease of reference.
It is apparent that either an error has been made on your part or my identity has been fraudulently used by a third party to order the [details of the goods/ services].
As I am not a party to the contract and have not ordered the [details of the goods/ services] in question, you have no legal basis for making any demand for payment from me.
I would request confirmation from you by return, that no further demands for payment will be made and that your records will be amended accordingly.
I look forward to hearing on you in the next seven days.