Graham.
I think I have an answer that will satisfy my need for viewing the sail well enough on a moonless night. I have bought an LED sternlight, 1W power, and mounted it on the shallowly sloping forward end of my coachroof, at 90 degrees to the normal angle, that is, so that the usual 135 degree angle of visibility is in the vertical plane, from horizontally forward, through vertical, to 45 degrees aft. There will be some light shining to either side of this, of course. It will reflect off the bottom of the sail bundle when anchored or moored, and illuminate the whole sail while sailing.