For amounts under £3,000 HMRC will send you a P800 tax calculation and normally collect the unpaid tax by reducing your PAYE tax code 'in year' and for the next tax year. For larger amounts or if is not possible to collect via a tax code they will send you a Simple Assessment called a PA302 showing what you owe and how to pay.
If you have received A PA302 payment in full is due by the 31st January after the end of the tax year. For example, tax due for a 2017/18 PA302 will be due by 31st January 2019. Interest and surcharges being added for late payments.
For years up to 2016/17, if you didn't respond, they would have issued a tax return for the year(s) concerned and you would then have fallen within the system of self- assessment (SA). The full payment being due by 31st January after the end of the tax year, with penalties for late filing and interest and surcharges being added for late payments.
HMRC have removed the self assessment burden for people who are only in it because their state pension is higher their Personal allowance (£11,850 for 2018/19, £11,500 for 2017/18) and it is the only way the tax can be collected. HMRC will issue a simple assessment PA302 which you just need to check and pay by the usual deadlines. If you are concerned about what is happening contact either HMRC or Tax Help for Older People.
Posted in: Problems paying your tax