For those of you who have to do tax returns and are expecting to need to pay tax this month. If you don't then this is irrelevant for you.
If your tax bill is over a certain amount for the year to 5 April 2018, then, unless the tax has been coded out into the 2019/20 tax year, you also have to make payments on account for 2018/19 this month and again in July.
I have discovered that several of my clients have been sent tax statements which have missed out the first payment on account for 2018/19 and just include the balance for 2017/18.
I've been in touch with HMRC who are aware of the bug and they are trying to sort it out.
It seems to relate mainly to people who are on the PAYE system for wages, salaries and pensions rather than those who are not.
If this applies to you, you may need to contact HMRC to sort this out.