[In the course of posting this question, I figured out how to do this. Sometimes the effort of formulating a question clearly so that someone else can understand it is all that's needed to find the obvious answer!! I'll post anyway in case someone has a similar difficulty]
I'm not managing something which I hoped would be simple enough:
Copy an object from a layer called "coloured"
and paste it to a layer called "background" without otherwise changing its location,