Incorrect P45

My pension provider has issued an incorrect P45 showing that I received £50 more than I did. They have also reported the incorrect figure directly to HMRC. They say they can't issue a correct P45 but have sent a letter which includes the right figures. How do I get this changed?

Hi Jay Sear,
While your pension provider may not be able to send you a corrected P45, they should still have sent us details of the correction made.
To allow us to confirm that your record has been updated correctly, or advise you further if this correction has not yet been received, contact us by webchat or phone here:
Income Tax: general enquiries
Thank you.