Unfortunately if the single yellow line was there and the CPZ entry sign was also there to be seen, the contravention is made out and as I'm sure you'll know, the fact that you didn't see or notice them is no defence.
You could make a plea for discretion.
The tactic of last resort would be to argue that the distance from the sign to the car was too great, and film a video to illustrate the point. We stick the video on youtube (as an unlisted video) and include it as a link in the representation, we can then use the view count from the youtube dashboard to confirm whether they've bothered to open the link and if they don't, we've got them for a failure to consider.