Alternate blank pages with header on export to pdf file from rdlc

I'm using rdlc reporting and have a header and footer.
If the header & footer contains text the report prints to pdf correctly.

For example:
If one text box in the header contains the text 'ABCD' then it prints to pdf correctly.
If I change the expression of the text box to be a parameter and set the parameter to 'ABCD' then in the pdf file
page 1 contains just the header & footer,
page 2 contains the header, footer & body (1st page),
page 3 contains just the header & footer,
page 4 contains just the body (2nd page),
page 5 is blank,
page 6 contains just the body (3rd and final page)

All the help suggests changing margin & page sizes etc. But the only thing I'm doing is changing from text to a parameter. It also goes wrong if I change it to a field in the database.

The report displayed in the report viewer looks correct - it is just the pdf file that is wrong.

Any suggestions?