Within the link I gave you, there is another link:
https://www.gov.uk/check-if-you-need-tax-return
Complete this one, and then, if you still don't agree with the result, I'm afraid you will have to steel yourself to wait on the phone to speak to them.
What was the basis for your appeal? You seem to be confusing self employed and self assessment.
You don't need to be self employed to be required to fill a SATR - a self assessment tax return.
I'm not self employed (and never have been), but I have been required to fill in a SATR for as long...