Thanks for the C&C and edit Eyecraft!
I agree with you about the background. I was thinking about some kind of dark space crusade / space hulk background tiles, but the grey is a little too dark.
However I tried sub-pixel work in the face and I don't agree with the mouth c&c. In the reference pictures, the mouth shape is not just a single line but a vivid dark arch. I think the subpixel doesn't convey that and makes it look like a dark thin mouth and almost a nose. As the sprites would be used at x2, x3 or x4 (for an ST/Amiga game) then I am also reluctant to use sub-pixel work for very key details.
I will have another play! BTW: Which one is the assault marine? Is it the terminator ones?