Masked edge of plane showing in export

Hi There
This is my first call for assistance with Hitfilm as it usually does what I want it to do. I don't know how to get around this problem however and I've not found anything by searching the forums.
Here's the story; I'm using the same techniques shown in the 'make a space nebula' tutorial to create a 3d scene from a set of planes hosting masked 2d images. My sky image is on the plane furthest from the camera - then a distant mountain image (with it's sky masked out) is set closer to the camera. As the camera pans down the distant mountains appear to rise against the cloudy sky - simple right?
The problem is the edge of the nearer plane (which is hidden by a mask) renders as a flickering white line in the exported video - which sort of kills the illusion!
I could get around this by creating a custom image in a larger format so that no plane edges cross the camera's field of view but I'm wondering if there is a simple solution that I'm unaware of.
Here's a screenshot showing the offending line. It's a simple scene, cloud image on far plane, mountain image  with masked sky on nearer plane and a 'Snow' particle effect.
Any thoughhts appreciated.

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