Had this been appealed initially within the appeal period, this appeal would have been successful and APCOA would have cancelled it. At this stage, they may decide not to. However, that is no big deal. APCOA are benign and will never resort to litigation.
Even if you did nothing and simply ignored all the debt collector letters, they would eventually stop and that would be the end of it. They rely on the low-hanging fruit on the gullible tree to capitulate and pay up out of fear and ignorance of the law and their rights.