AT JustDave Does it do it without the texture packs installed, or any other problems with the demo? Perhaps the file was corrupt when you were downloading it if so. I actually had similar corruption with the White Scars skin, I had a weird overlay over everything and it looked like it was actually the texture for the White Scars skin itself.
Could try scanning the game cache on steam, redownloading the skin, or perhaps updating your video drivers? Those would be the first things to try I suppose.