Was re-reading some of Jurgen Wolff's excellent 'Your Writing Coach' again today. He suggests that early in plotting (before character profiles) it is useful to interrogate yourself on the 'Why?'s of your idea, and to keep questioning until you have more insight, and also perhaps other avenues of the story.
After this, he advocates using some 'What else could happen?' questioning. It was delicious opening a brand new notebook. I have quite a little stash of them, most of which I'm slightly reluctant to use - they're so pretty when they're pristine, before I've filled them with scribbles and post-it notes. But I carefully wrote a title and the questions above. And then I closed it again.
I will come back to it - maybe even tonight - but first I had to take the children swimming, and make turkey burgers with them, and listen to them read, and cuddle the little one and read to him, and then I fell asleep on the sofa in a late-pregnancy induced stupor. But that notebook is like a promise to myself- planning this as a career, not just another dabble at a story.