The Traffic Management Act 2004 introduced powers that allow an enforcing authority to serve a postal PCN when a PCN was in course of preparation, but the driver returned and drove off before the PCN could be served at the roadside. This means that the PCN has been correctly served IMHO.
So, looking at the actual alleged contravention, would your claim of an emergency in having urgently to stop for a wee-wee, and thus park on the double-yellow lines. Well, I don't think this would be a runner, unless you can show some medical evidence of a medical waterworks problem. However see what others say.