Check your tax code!
When you get your payslip for April please ensure you check your tax code.
The new standard tax code will show as 1150L. This means you can earn £11,500 tax free. Your tax code will be reduced if you have multiple income sources, such as the state pension or other employments.
If you think your tax code is wrong, it is better to get it sorted at the start of the new financial year, rather than letting possible problems build up over the year resulting in a bill for unpaid tax.
HMRC is very clear. Even though it issues the tax codes, it is your responsibility to ensure that your tax code is correct.
See: GOV.UK: Tax codes (external website), to query your tax code.