Are you just trying to get it to repeat cleanly?
Is it possible you just need to crop the image you're defining as the pattern so that when it repeats the pixels at the one edge exactly match up with those on the other?
Yes thats right im trying to get it to repeat horizontally cleanly.
The original image I used to define the pattern was exactly 20pixels by 8 pixels, the triangles top filled the image and the point of the triangle reached the very edge of the bottom of the image so there was no gaps.
Here you can see the image im using to apply the pattern and even in the preivew you can how its repeating vertically:
Patterns repeat horizontally and vertically. That's what they are supposed to do.
The only ways to make your pattern repeat only horizontally are to scale it so it fits your shape vertically, or trim the shape so it fits the pattern vertically.