Whatever I change in the settings I can't get the other boxes of the predicted deprivation section to be place under the group title. Ideally I would like to have 4 columns UNDER the group title with 2-2-2-1 boxes. Any ideas?
I don't think this is possible within QGIS. So I suggest you export your map to Inkscape and edit it manually.
Once you're done with all other things you want in the print composer , export your map as SVG then open the SVG with Inkscape.
Inkscape is a free and open-source graphic design software. So it blends seamlessly with QGIS.
You can download and learn more about Inkscape here: https://inkscape.org/release/inkscape-1.0.1/
If you're absolutely new to Inkscape, this tutorial can get you up and running in a few hours: https://www.youtube.com/playlist?list=PL__WA5iiRT53V2UobbmFvzAW_-LJMkPIV