I don't recommend the SDS on this problem if you have a sprained right ankle, I was writhing on the mat for some time after my first proper attempt at it :-(.

Is the grass sloping in the background, or is it just a squint horizon?

The boulder is on a hillside (see the helpful crag location map, which you can view by clicking on the hyperlink below the photo and then on the crag name), so the grass is sloping.
Also the boulder on which the mat is lying is sloping (both right to left and away from the camera), adding to the general impression.
As evidence of correct orientation, the cloud strata in the background are pretty level aren't they? And the buckle on the mat is hanging vertically as it would unless Newton got it wrong all those years ago.

Oh and you can see the angle of the overhanging arete (if from the other side) in http://www.ukclimbing.com/images/dbpage.html?id=98563 - it's the one on the LHS and Jenn is sitting on the boulder which is supporting the mat in the above photo.

Of course it could all be an elaborate hoax, like the Apollo moon landings...