Manadatory reconsideration for PIP appeal question

I had my benefits downgraded when switching from DLA to PIP three months ago. I asked for a mandatory reconsideration and the decision was upheld, as apparently is usually the case. I have put in motion an appeal but am outwith the month limit since receiving it so it will be late, as I got confused and thought it was another call to the DWP to appeal.

I know someone who sent an appeal in the form of a letter explaining what was wrong with the decision in his eyes going by the report of the assessor, and as a resulyt he had his money increased. However, when I look for advice anywhere it seems to be that you just fill in the application form saying why you are appealing and thereafter wait to hear back about a tribunal? I can't see any accounts of people writing letters at this stage, seperate from the appeal form, which would have the potential of getting the decision overturned. Obviously I would rather this than go to a tribunal months down the line.