Is the Brooks clamp ID matched to the seat post OD? Part of the mechanics of clamping is dependent on the "ears" contacting the post and no longer moving (slack). If the OD is too small, the clamp ears will not "bottom" out causing the rail clamp not to reach required clamping force to prevent rotation. The other issue is that if the saddle has moved often enough, the "teeth" of the mating surfaces are now round allowing for rotational slip. Adding a gritty surface between the two might help but would be marginal IMHO.

WRT 2. A sleeve would work too, but now you need to use trial and error to determine the correct length that will allow for sufficient clamping to the post before allowing for increased clamping to the rail.

Maybe you just need a replacement post or Brooks clamp.

Is this what you are working with?
[IMG]OLYMPUS DIGITAL CAMERA by superissimo_83, on Flickr[/IMG]