Previous Employer submitted incorrect PAYE information leading to tax overpay

Hi, My previous employer during my employment submitted incorrect earning data, leading to a change in my tax code leading to an overpay in tax myself and my employer went back and forth with HMRC regarding the resolution to this and then my employment came to an end and now they are unwilling to make an effort towards rectifying this and ACAS are no help and I cannot access my HMRC account due to an “MCI ERROR” How am I supposed to correct the information if my employer will not act and ACAS cannot advise via tribunal ?

Hi,
If you have a MCI error on you tax account, this usually means some sort of changes are being reviewed by HMRC to your employment record(maybe linked to your issue but also maybe not), unfortunatley this forum is for general questions not specific individual queries,
If you contact our helpline and advise you have a MCI error on you account they maybe able to offer further information Income Tax: general enquiries
Thank you.

The general enquiry, regardless of personal details is contained in the message above, how does one get a modification made to their annual taxation records if the company/account that submitted the records have submitted them incorrectly and do not want to resolve the issue ?

Hi,
Normally the employer should amend the incorrect pay and tax information on payroll and this then updates your HMRC tax records automatically. If you are unable to get them to make the updates please write to us providing evidence of the correct income details so we can review this further. 
Income Tax: general enquiries
Thank you.