YOu would seem not to have attached the actual PCN, which we need to see, all sides of it, so please do so. From what you say, it is a Reg 10 PCN, served by post because you drove away thinking you'd get away with it. Not so, unfortunately, since the Traffic Management Act 2004 came into force.
The double-yellows also have double kerb-blips indicating No Loading 24x7, so you may struggle with this one