Unfortunately as seen on this forum, PCNs served to vehicles do go astray quite often, envelope and all, but the photos show a yellow PCN envelope on your car. So the absence of a PCN inside the envelope means one of two things, a little ned opened the envelope and pinched the PCN, OR the CEO never put the PCN in the envelope. I suppose this can happen but it's difficult to prove and hinges on who is believed, you or the CEO. The council will always back up the CEO, so you'd need to take them to London Tribunals. On the other hand, they may well reoffer the discount, and this might be your best option. It all depends on your attitude to risk, as the full penalty will be in play at London Tribunals if you take them there. However, that is all that is in play, there are no additional costs, and if you win, you pay nothing of course.