The A23 has several miles of roadworks with a 40MPH limit and Average Speed check with ANPR cameras northbound. I have not seen any southbound.
Anyway, travelling north today and having passed the cameras, I was aware of traffic entering from the left a bit further on.
So, is there any check on the later traffic, or are they free to go - except of course barring the occasional police vehicle?

Average speed works by measuring the time it takes you to drive the distance between the first and second set of cameras. If traffic joins and doesn't pass through two sets, I'd have thought they wouldn't be measured.

That's what I thought - was hoping someone knows the answer

You have to pass between at least two cameras, it's like start and stop lines.