There is a known issue with HMRC's systems that is affecting some taxpayers who have filed their 2017/18 tax returns and have 'payments on account' to make. A payment on account is an instalment for next year’s tax. They do not always apply; it will depending on the level of your tax liability and how much tax you have collected “at source” (from your wages or pension income, for example).
It sounds as if you don’t use an accountant or tax adviser, in which case you should check what your 2017/18 tax return said when you filed it. If you find there is a dfference, you must contact HMRC to put it right.
If you’re using a tax adviser or accountant, make sure that the amount due on any HMRC correspondence or when you log-in to your tax account, is the same as what your adviser told you it would be. If there is any discrepancy, make sure you call your accountant or tax adviser; you should not rely on the information being sent by HMRC.
If you are affected by the issue, you must take action because you are at risk of HMRC accidentally reimbursing the 'overpayment' or being charged late payment penalties if you pay too little.
If you'd like any assistance with this, please feel free to contact your local TaxAssist Accountant.