Error in tax returns filed by Accountant

I got letter from HMRC saying there is discrepancy in the returns and tax paid for the year 2016-2017. This was filed by our accountant. Having had a close look I noted accountant has accidentally put income from my husbands P60 and his NI number. Hence I have been overtaxed for that year. My accountant asked me to phone the HMRC to correct this , Telephoning the HMRC and wait of 57 minutes on phone they have asked me to have web chat or call after September 4th !!!

Hi VBAB P,
Our Self Assessment telephone lines are closed until 4 September.  
You will need to contact our Se:lf Assessment webchat helpline for advice here:
Contact HMRC
Thank you.