SSRS invoice totals

Hi,

  I'm redesigning the SalesInvoice SSRS report and trying to display the Pre-tax amount, Tax amount and net amount.  Inside a tablix these fields sum correctly but if I create a text box and add the expression =Sum(Fields!TaxBaseAmount.Value, "SalesInvoiceDS") the total is different to what is displayed as the tablix (=Fields!TaxBaseAmount.Value).  In fact the total outside the tablix is exactly double the total displayed inside the tablix - it seems to be summing it twice.

  Any idea how I can get the correct invoce totals to display outside the tablix?

Thanks,

David