Graham
I remember that I had issues with the header way back in version 3 or something. Here are some things that may help:
On the "COMPANY" options page -- I have a report logo height as 0.75
On the "INOVICE" options page -- I have a logo height as 0.75
On the "Other" options page -- Margins and other report settings as : Side 0.75, Top/Bottom 0.75; Margins small are 0.25 for both.
If I remember correctly, I fiddles with these numbers and my logo and it finally all fit back then -- and after that I had no more header issues.
Hope that helps.
Ken