I've taken advice provided in a previous answer which has solved my original problem - avoiding "Series1" appearing in the legend.
I have also stored the group colour in the database, so that is now retrieved as part of my query and used in the series Fill expression. This too is working as you can see by the bar colours in the attached chart.
My problem now is the legend seems to have the correct colour for the first group in the legend, but all of the others are black! See the attached screenshots that show the data that is being returned by my query, and a chart of the question. As you can see, the SME group was assigned to the NULL answer and with the same colour assignment.
Any thoughts on this issue?
Thanks for all of your assistance.