If you did not listen to Money Box today you may not have heard that HMRC are admitting that some of their records are in a state.
Clare Merrills from HRMC said "must always check anything that comes from us, including your tax code, every year."
These links to articles may help you check your tax code:
MoneyMagpie - Check Your Tax Code
HMRC: Check you're paying the right tax
Understanding your PAYE Coding Notice