Need a tax refund and employer not helping.

I worked on a temporary cintract for 3 months over christmas and as I am under 18 and earn under the 12,500 threshold I should not be paying tax however after switching to a full time contract, i received a p45 however was not advised to give this information seen as it's tecnhnically the same employer. I have been on tax code 142t for the most recent 2 months and have been paying PAYE on that as well and employer has said it will get sorted however it hasn't and i am unsure on what to do now.

Hi,

You will need to contact our Income Tax team to review and update your tax code.

Income Tax: general enquiries

Thank you.