HMRC Admin 5 Response

Hi

You are correct that this only works with original submissions.  Where a tax return is amended to include a loss carryback, then a manual intervention is required.  
The is dealt with by the taxes technical officers in the customer services department, who will need to calculate how much tax relief is due.  
You will need to contact our self assesment helpline on 0300 200 3310 or contact our webchat facility at Contact HMRC, to have the amended loss carryback calculated and a credit added to your self assessment statement.

Thank you