Because some insurers now ask about courses in the last 5 years, I would personally challenge it in Court (with appropriate legal advice) if the picture shows a blank gantry. Because these things need proof reading manually to check the gantry picture indeed showed the correct limit, I suspect it's a mistake issuing it. This occasionally does happen, e.g. bus companies getting penalties for driving in the bus lane...in a bus.
One should never plead guilty to an offence one has not committed.
I wonder if, if the photo link doesn't work, it's already been cancelled? Is there a phone number you can call on it?
Thanks for the suggestions.
I've managed to get into the camera images now. There's a clear picture of my car clocked at 47mph, and a picture of the gantry showing 40mph.
The info also states that the signs had been up for 2min 50 sec. So having gone back and forth on Google Street View, I can only conclude that the "blank gantry" I mentioned was the one before, and the gantry where I first saw 40 mph was two before the one where I was clocked.
The time for which the limit had been up would suggest that although the first gantry "came on" as I approached it, the third one where I was clocked had been showing the reduced speed for a few minutes. I was clearly slowing down anyway, just not enough!
This also shows that if a gantry is showing a reduced speed when it first comes into the drivers view, then you need to reduce your speed to the indicated figure by the time you reach it, or face getting clocked.
This is just like the stereotypical copper hiding behind the 30mph sign, radar gun in hand as you enter a town/village from an A road.
So I think I need to take this on the chin, frustrating as it is.
For those who are interested in such things, the first gantry I passed (just after the M20 junction) was no 138/S1, the second (blank when I passed) was no 124 (adjacent to lamp 4Q404) and the one where I was clocked was number 116/S0