The Road Angel warns of these Specs cameras as well as all fixed and temporary speed cameras. The only thing you still have to be wary of is the mobile vans.
The speed camera signs are used only where there is a fixed camera or they frequently use mobile vans, so if you see a sign but no camera it is likely that there is a van working that road every week or so.
As far as I am aware Blackspot do give a £50 reward if you store a camera location that is not on their database. They tell me they have never given out any rewards as they always get the database updated quickly.
It could be the cameras that were seen on the M5/M42 are not active?
Local authorities have to get planning permission before erecting a speed camera, so the information is readily available. Blackspot have a 'relationship' of sorts with police and local authorities.
In six months or so and just over 20,000 miles, I have never passed a camera without my Road Angel warning me (as far as I am aware!!) I have had a few 'false' warnings where temporary cameras have been or are about to be sited.
'Stick to the limit and you wont get a ticket!'