Joe, thanks for that. I'm certainly not keen on loosing any of the image, and since I'd want to stick to constant-height I would have black bars top. bottom, side and side in 16:9 and 4:3 too, assuming that shrinking the picture to that extent would even be possible. I don't like the thought of paying for all that screen and then not using a good proportion of it, so a 16:9 set is clearly not the way to go for me.Unless you were willing to Zoom and loose some image you are stuck with black bars top and bottom (2.35:1) or side and side (4:3).
I'm coming more and more to the thought that what I need to do is get the projector accessible from the room rather than only from the loft. At the moment though I can't see a neat way of achieving that.
I have wondered about the possibility of a long-throw lens which would let me mount the projector against the back wall rather than in the middle of the room, but I haven't looked into the idea in any detail - and even if some kind of add-on were possible, wouldn't it, and the increased throw, cause a significant drop in brightness?