It arrived this afternoon - since I didn't want to go out again, I just took a few pictures through the windows to see if it worked and haven't done much beyond that. I didn't even use a tripod, and held the filter square over the lens by hand, and I'm surprised at the quality of the results I got. Exposure was 1 second at F8. I've taken the two pictures I liked best and produced three images - one as taken by the camera, one with contrast equalisation, and one after conversion to black and white. I know that there's a lot more that can be done to improve the quality and impact, but as I've said this is just a quick test. In view of the slight shakiness of the images I've hosted them on Photobucket, if anything the size reduction makes them look better.
Next step I think will be to get a Cokin or similar filter holder, hand holding was a bit of a pain, and take the camera out with a tripod to see what it can really do. After that I'll decide if I want to spend more and get a better filter.
Incidentally, some of the tutorials I've been looking at have been recommending 8 or 16 seconds at F8 for other cameras; while I haven't tried other settings yet it looks as if 1 second is OK with the Nikon D50, which will help immensely.
Later - forgot to say that the tree on the right in the first picture, nearer the camera than the others, is actually holly, an evergreen - it looks white because the leaves reflect infra-red very effectively.