Here’s the VMF
The material I tested with is completely default, nothing linked up (no base colour etc) although the problem is still there with other materials. Here’s how it looks with the default grey dev texture: http://imgur.com/7p9OXmI
Not quite as obvious but still has very visible edges.
Maybe I’m not running the latest HammUEr? I downloaded what I thought was the most recent version, deleted the old, and added it in the plugins folder.