Thanks all for your suggestions. Sorry for my delay in responding - this is a weekends-only thing for me.
No Solver it's not an existing structure but the image shown in Jo-Ann's post is exactly what I'm after.
The Clerestory KBA method is working for me, except that I'm having trouble getting the elevated bit of wall which joins the two roof pitches to work correctly or at all. (That is, the wall above the left hand one of the two inner temporary walls.)
Jo-Ann, you've solved it but I don't know how. I may be not using the forum controls correctly but all I can see of your post is the last 2 lines and two thumbnails. Tantalising!
How did you get the wall to draw?