Odd question but were there 2 officers there? The reason I ask is because the "their word against yours" situation is something that the courts try to avoid at all costs (bad image, revenue raising etc)

The last time I had something like this happen to me (one cop) for the same thing (stop sign) I asked him how he could be sure seeing as there was only him and no back up). He gave me the ticket anyway. I sent a statement in and they withdrew the offence.

I'm not sure if you can oppose a fine in this way anymore but this was about 7 years ago.

Anyway if there was 2 there's not much you can do unfortunately.