Difficulty printing borderless photos for custom sizes

I am finding it difficult to print a borderless photo. When I try to print using the photo paper I have it always prints a border (or does not print in the border - so i guess this is called cropped).

The photo paper is not in the "Paper Size" list of the print dialogue in Preview (or iPhoto). So I specify the size using the "Manage Custom Sizes" option (similar in iPhoto). The size is 89 x 127 mm. In the print preview it looks fine, but when it is printed, a white border (ie, non printed area) crops the whole image. The one side is a 5mm border, with its opposite being 3 and the other two being 3.4.

I have noticed that when I play around with the Paper size, the standard sizes (like A4) for example, all have a "borderless" option (ie, I can select "A4" or "A4 (borderless)"). When A borderless size is selected certain other options are disabled/enabled. For example, the options under the "borderless printing" 'tab' is enabled and the options under the "duplex printing and margins" 'tab' becomes disabled. This is the exact opposite for when I try and specify my paper size. So perhaps this is a factor. (Even though I would have expected the "border" option set to "none" would mean that there would be no border...)

So just to save ppls time, this is what I am doing when I print:

1. The image is exactly 89x127mm (created with GIMP)
2. I print using Preview (similar results are achieved in iPhoto, but more pain to get there)
3. I set the paper size to 8.9 by 12.7cm (see note 1)
4. I don't change the scale (though i have experimented with all options here and nothing seems to change).
5. I check that Layout > Border > None is selected. It always is.
6. I change Quality and Media > Media Type to Glossy Photo Paper. (not that this should matter)
7. I go to borderless printing tab and note that it is disabled. great.
8. The preview shows no borders and so I hit print, but the borders come anyway (the image gets cropped).

Note
I have managed to print borderless photo images with this printer using the supplied photo paper that came with the printer. I don't think i have been able to do it since moving to this new size but I may be wrong...

Note 1
When I am setting the size in Preview, I can specify the "Non-Printable Areas". I set these to zero, though I note that the 'canon mp640 series' presets are the exact measurements that is the area that does not get printed.

Note 2
In case it has a bearing on the problem, the printer I am using is a Canon MP640.