MFratson

Hi, I set up a payment plan in January on my Self-Assessment, and it has been working successfully. Payments are being made and received monthly. Yesterday I received a letter telling me that I owe £330 and it is late, and must be paid. This is obviously incorrect because I have a payment plan. So why am I being told that it must be paid now all at once? I have tried calling over the phone, repeatedly, but get useless information and direction to irrelevant information I've already read. It's incredibly frustrating and I can't help but think a complete waste of time. Please could you advise?