Smart will ignore you, or simply tell you it’s too late to appeal and your appeal was rejected anyway.
It’s something you can use in your defence, in some way, when they take you to court. It’s likely that the case will ultimately be discontinued if you defend it, with advice here to help you. Personally I wouldn’t contact Smart to give them advance notice of a defence point now.
In due course Smart will lie and say that their PCN complies with the requirements of PoFA 2012, they have form for this.
If you’d used it as a POPLA appeal point, there’s a fair chance your appeal would have been upheld, which is why Smart no longer uses POPLA but the totally non-independent IAS which is in cahoots with them, but we are where we are now. The POPLA ship has sailed.
I wouldn’t hold out much hope for McDonald’s but it’s worth a try. They usually disclaim responsibility in one way or another.