Aggregate (sum) balance amounts by group in an RTF template (such as a Payslip)

I've been asked to update a BI Publisher RTF template to consolidate a list of Payroll Balances by Reporting Name and Hourly Rate.

The data in the XML extract appears in blocks like this:

This means, on the report, each Balance Value appears on a different line — even when the Hourly Rate is the same — because each entry appears in a separate block in the extract XML.

Instead, we need to list each combination of Reporting Name and Hourly Rate once, with the total number of Hours and Payment amount.

The complexity of the report tends to make it easier to use XSL throughout. But