Краткий справочник по деловой переписке

Dear Sirs,
May we remind you that our January statement amounting to $400 is overdue?
We should be grateful to receive your cheque at your early convenience.
Yours faithfully,

Dear Sirs,
We refer to our letter of 15th April in which we drew your attention to the overdue balance of our January statement of $400.
We must assume that this account has escaped your attention and we should be glad if you would look into the matter without delay.
Yours faithfully,

Dear Sirs,
We have rendered our statement for your January account three times have asked you for settlement the overdue amount of $400 in our letters of 15th and 29th April. We are surprised that we have not even had reply to our letters. No item of account is in dispute we must now is in dispute and we mast now insist on an immediate settlement. Please note that we shall have to hand this matter to our solicitors if your cheque is not received by 20th May.
We need not tell you how much we should regret such a step after the long and friendly connection with your firm and we hope that you will help us to avoid it by giving this matter your immediate attention.
Yours faithfully,