The restriction that a double yellow line indicates is that parking, (actually "waiting" in law), is barred on the road 24x7 , and this includes not just the carriageway, but verges and footways up to the property line, in this case the stadium wall. So by parking on the footway, you were still in contravention. We see a lot of cases like yours on this forum where the driver does not know about the meaning of the restriction. A single yellow line imposes the same restriction, but only on the days and hours indicated on a sign.
So I think that you having received their rejection letter, there is no point in taking the matter further unless somebody on here suggests a robust 'technical' appeal based on council mismanagement of the enforcement process, or the PCN contains fatal errors of content making it void. Have you posted all sides of the PCN so we can check for errors ?