I fixed this problem
On my girlfriends car once, dc5 base model though, she had one yellowing headlight i used Some medium cut and polish to cut back the top layer and then a lighter and finally turtle wax scratch and swirl remover, this combo worked for me and brought it up looking alot clearer then before I did it.... And also on my dc2r I sometimes start to get a little haze at the top of my headlight but a nice rub with meguiers scratch x seems to clear it back up also..
Hope this helps a little..