Your failure to respond to our letters of … and … is quite disappointing.
Your failure to settle our invoice no. … despite repeated reminders will no longer be tolerated.
After three reminders, we urge you to clear our invoice no. … without further delay.
There can be no question of further extension of payment.
We must insist that you settle the outstanding amount within seven days.
… otherwise we shall be forced to place this matter in the hands of our solicitors / legal advisers.
Failure to comply with this will result in debt collection.
We trust you do not wish to jeopardise our trading relations.
We must urge you to deal with this matter right away.
Thank you for your prompt attention.