Layby sleepover - Laws?

IamtheGaitor

I know there was a recent thread on this but of course I cant find it when I want it!!! 

I recall people stating that you couldnt overnight in a layby and that they had, or knew others who had, been moved on. What I cant find is a specific law they are breaking as HGV's do it all the time. 

Other than the laybys that have signs that say ''no overnight'' what is the law that prohibits it?