Ok that seems to have worked by inserting the formula
pageNumber Mod 2 = 1 in the Group footer and removing all reset page ticks from other sections. Additionally I have also placed this formula as condition in Surpress Section to save on spurious blank pages.
But now I have another problem:
Previously the report had an indicator Page N of M that I could use which gave the customer an indication of the pages and page number in the Invoice. Of course the count had to be reset so that each new invoice detail started at Page1.
Any ideas how I can achieve this?
Many thanks in advance