Thanks Andrew, theres lots of good info here
I was trying to change some aspects of the assignment grader table, so it works better on low resolution/small screens - hiding unnecessary columns, wrapping file submissions etc. As you suggested I got there with classes in the end.
I will need to look into some of the functions and spend some serious time on YUI tutorials but its always good to have something to work towards and I was testing subpages (an OU development) recently and found that they havent included the activity chooser, I assume this is because they dont use Ajax rather than due to any other difficulty. It was raised as an issue but they keep closing the tracker because the issue doesnt affect them.
Anyway, I would be feeling very smug if I managed to make the necessary tweaks in the not too distant future - Ill be sure to visit the forums more often!