Dear Sir
We do not appear to have received any reply to our previous requests of _____________ (letter 1),_____________ (letter 2), _____________ (letter 3) and _____________ (letter 4) for payment of the sum of Rs. __________________________ still owing on your account.
It is with the utmost regret that we have now reached the stage when we must press for immediate payment. We have no wish to be unreasonable, but failing payment by ___________________________, we are afraid you will leave us no choice but to place the matter in legal hands, but we sincerely hope this will not become necessary.
Yours faithfully
Image Source: cache-blog.credit.com/wp-content/uploads/2012/06/late-payment.jpg