[Insert Name and Address here]
Tip - Find out the name of the person responsible for payment of bills and address the letter to them. A copy to the Managing Director would also help (his/her name might assist also)

Your ref: 
My ref: [insert your invoice number ]

OVERDUE(Use perhaps in red on your second letter)

Dear [name]

Re: Outstanding Debt £[insert] dated[insert]

We/I refer to my/our above mentioned invoice which appears remain outstanding despite persistent reminders.

I/we cannot extend your credit any further and legal proceedings look inevitable.

I/we will allow you a final 7 days before handing the case to my/our solicitors. Time runs out at close of business on [insert].

Please save yourself the expense of court proceedings, solicitors charges, and interest charges, and pay now. 

If you have paid your invoice recently please accept my/our apologies for sending you this letter

Yours faithfully, 

[SIGNATURE]

[PRINT NAME]