Thanks for the suggestions - I'll crack on with the trail & error I guess!
The SQL is:
SELECT QryRatesCheck1.Phase, QryRatesCheck1.StaffNumber, QryRatesCheck1.StaffName, QryRatesCheck1.StaffAgency, IIf([StaffAgency]="Staff",([StaffInvoiceRate]),[AgencyInvoiceRate]) AS RateA...