Graeme, on the back wall about 2 1/2' behind the listening position should be excellent. My suggestion would be separating them so as to be just 6" or so from, but not in, the corner and facing directly forward.

Since there's so little room behind you to the wall to form a rear soundfield, it's not likely that using one(or preferably two)back surround speaker in essentially the same plane as the QSs would be of significant benefit. You can, of course, experiment with a temporary setup.

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Enjoy the music, not the equipment.