Hi,
I would create a separate dataset and table for this which returned the averages for each Area Group. From this dataset you could work out the District Average.
When the values shown in this new table are correct you can hide the table and use the reportitems function in the report to place the value in a cell in you main table.
The reportitems function allows you to copy the value returned in a cell to another cell.
The formula is =reportitems!FIELDNAME.value
I am assuming that you will only one district average?
If you have multiple district averages you will need to use a different approach.