Driving along the M25 a few nights ago I was flashed twice within 10 minutes! first was in a variable speed limit of 50, I certainly wasn't doing more than 50 and about 10 minutes later was flashed again when national speed limit was showing, obviously due to the earlier flash I was 100% doing under 70, no cars were overtaking me on either occasion, question is do motorway speed cameras sometimes flash at random as a deterrent or do I expect to receive 2 NIP.
I drive on the north part of the M25 pretty much daily. They are very sensitive, ie just a few mph over the limit and they will flash (not happened to me personally, but have seen others be flashed when they are driving past me). They will also flash if there is a red X in that lane. I don't know if you will expect two letters, sorry. But in the past, I've been flashed on the M1 only a few mph over, and never received anything.
Sometimes they flash once I usually sit at 74mph on the M25 and have never been flashed On the M4 and M5 where they are smart motorways i usually sit at 72 mph again with no issues I have been flashed once on the M5 but no letter came through Just my experience though