hi Paul,
a group cannot unfortunately be sorted on anything that is considered whileprintingrecords...e.g. running totals, shared vars.
however, if this is a requirement then you can pass the sort criteria to a subreport and then use the subreport to display your data. the subreport is then sorted using this criteria (rolled up in a string running total) and the main report is suppressed.
as an example, have a look at the attachment...extract the contents and change the .txt extension to .rpt. there are some instructions on the report as to how to use the above technique.
your other choice is to create the report off of a Command object and create the running total in the command object.
SQL Expressions can sometimes be used (depending on your database) to bring in RT data but require a Select statement inside the expression which is unfortunately not supported. so a Command would be recommended should you wish to do this with SQL.
cheers,
jamie