Try defining a calculation field: GetSummary ( YourSummaryField ; your "sorted by" field )
and export it for each group.
I'll come back to that and give it a go.
As it happened trial and error revealed the issue.
As you'd know Export has three boxes when grouping.
Field to select, field to group by and fields to export.
What was happening was that I was selecting fields to group by after fields to export.
Curiously the Group By has to be selected before selecting the field to export. It then works as expected.
Wasn't very clear in the documentation and starting with Excel Export from Prview mode is misleading.