customer aggressive when chasing debt and refusing to pay

Hi guys. New here. Your support is greatly appreciated.

Our payment terms are 14 days and we advise customers on email or verbally of this and it is stated on our invoices.

To date most customers paid on time but, 1. Each time we chase debt we get silly excuses like, we sent a cheque but, it doesn't arrive for weeks with us chasing them via email.

Lately, they owed 2 invoices, paid 1 later but, not an older one. Chased this got told it had been paid but, wasn't. Chased again and told we don't care what your terms are and they've never been agreed ours are month end plus 30days and we'll never do business with you. Fair enough but, our terms are clear and in the past you paid when we chased payment. Now you've decided to delay payment, 60 days now.

I have emailed 4 times and now preparing a letter before action requesting payment in 7days or commence a small claim as this is just ridiculous. Emails replied are worded, your accounts is the worst you're all rude and arrogant, you sent silly emails over payment...

We're not in the business of chasing debt so, this is time consuming and despite our polite approach they've proved difficult.

Any advice?