Crosstab ordering

Hi

I have a Crosstab that groups by Product Category 1 & then by Product Category 2. There is one sum field of No of Units.

Currently the Crosstab sorts in alphabetical order by Product Category 1. I would like to sort the Crosstab by the Total Units value in Product Category 1 instead.

Any advice regarding who I might achieve this would be appreciated.

Thanks

Jon