It’s a Penalty Notice. Just let it time out after 6 months. Appeal to APCOA on day 27 as the keeper and tell them you paid. No more detail than that. Ask why are they pursuing you.
Eventually you will get a POPLA code when they reject your appeal. It will say you have 28 days from the appeal rejection date. The code is actually valid for 33 days so submit a POPLA appeal on day 32.
The ai m is to string this out as long as possible. APCOA are benign when it comes to litigation and the odds of them pursuing a private prosecution in the magistrates court are about the same as you winning the lottery.
Even if they were to be so stupid as to try and they won, they would not get a penny of any fine issued as it would go to the public purse and not to the thieving bar stewards.