AdvancedDataGrid ItemRenderer for Group Column

Hello

I have an ArrayCollection of 'Lessons' which has the following fields:
topicName, lessonName. The lessonName is unique for every row but the
topicName is not. I want to display the Lessons grouped by Topic as follows:

Topic 01
Lesson 01
Lesson 02
Topic 02
Lesson 03
Lesson 04

So far I have created a GroupingCollection2, used this as the data provider
for an AdvancedDataGrid and in its raw form this works.

Now I want to style the columns. I have been able to create a
MXAdvancedDataGridItemRenderer and set this as a
AdvancedDataGridRendererProvider for my Lessons column. By setting the Depth
to 2 the Lessons are all styled as I want.

How do I create an ItemRenderer for the group headings in the Topic column?
No matter what I try either it has no impact on the display or it renders on
every row, not just the group headings.

For example I tried this within the <mx:columns> but it renders every
column:

I have tried creating, and referencing, an AdvancedDataGridGroupItemRenderer
but this didn't influence the displayed data:

Any advice would be very welcome.

Thanks.

Chris