Im going to replace my brake pads tomorrow and have noticed that my discs are slightly scored! What would be the best grade of sandpaper to use on them? Im guessing they should be completely smooth?...

Depends how bad it is. I've scored them very badly on the transit before and just put new pads on, they smooth it off again Photo would help with an answer.

I really wouldn't bother, check the disks are within the minimum thickness spec (11.8mm 1980 on) at their thinnest part and renew if necessary otherwise wait until they are worn out.

You can get a pair for £20 it seems.