Ha ha. Indeed not.
Don't ever let anyone talk about herding cats - herding baby chicks is much harder.
I managed to persuade them to go under the heat plate just before dark. They can easily get out, or get partially out, so I don't think they will get too hot. But for now, all is peace and quiet. I know I won't be able to help myself and will check on them again just before going to bed, but I am much more relaxed about it having read all your responses, so thank you!
Ha! Just checked my camera and two of them have stuck their heads out from under the cave - but their bodies are under so I think they are where they want to be.