QTR-Quadtone RIP

I've posted this on Black & White forum, but as it is almost 
certainly something to do with qtr I thought I might get a solution 
here sooner.

I am making some sort of error trying to get a specific print size out
of qimage and qtr.
I want the print to be 744x480mm printed on canvas. I print to file
from qimage with a page size of 746x482 so that there is a 1mm margin.
When I have printed to file I check the print in photoshop and these
sizes are correct, so I can assume that the problem is not a setting 
in qimage.
I then load the file into qtr with a page setting of 800x610mm to 
print on an Epson 7800. The print is centred but not fit to page as 
there is plenty of room.

The print size comes out at 728x478mm (within the paper size of 
800x610)- much more reduction in length
than width. Could it be margins? 

Has anyone any idea what I am doing wrong?

Regards
John White