Incorrect Tax Code leading to overpayment of tax

How can I contact HMRC ( Not robot helpline) to explain and thus rectify an incorrect tax code (which means I will be paying an inordinate amount of tax)? This is due to a pension provider submitting figures incorrectly relating an uncrystallised pension lump sum which I have already paid tax on. HMRC have wrongly assumed that this is income prior to tax deductions and have given me a negative tax code for the year.

Hi,
You can contact us by webchat via Income Tax: general enquiries- just click 'ask HMRC online'
Thank you.