I assumed they were already live. I've been slowing down for the gantries with the cameras since they installed them.
And there's the flaw in the system. There are calibration marks under each gantry, but it soon becomes obvious that the cameras are mounted only on gantries that cover both carriageways, and in sets that point in both directions. The cameras on the opposite carriageway stick out like a sore thumb (even though they're painted grey) and so you know which gantries to slow down for.
And if you are caught, then you can probably get away with the same defence that you can use for the M25 variable speed limit - i.e. if you're clocked doing 70mph or less, you can ask for evidence that the signs were actually showing a lower speed - something that the authorities aren't in a position to prove.
I'm not sure if this loophole is still valid, or whether they've added some extra cameras to prove that the signs were working at the time specified, but it's certainly worth a try if you get a ticket through the post from either of these systems.
Cheers for the tip though! I'll be doubly careful through that stretch now