Ok, I haven't got D3d working with the original Unreal Tournament, but I have found out what was causing the problems with it running in Open GL.
Basically, when I had the sliders in the ATI Open GL 'Advanced Settings' tab set to Max Performance, I had problems with stuttering graphics and dodgy lighting even though the game was stating framerates of 100 - 150 FPS.
I then set the sliders to Best Quality, with all the AA and AF at max. All the GFX problems disappeared and the game looked great. Even better was the fact that the game was still showing framerates of 90 - 120 FPS.
Hope some others find this info useful and I'll post again if I sort out the D3D probs.