hi again,
I don't think it would do 5mm, however it would score the plastic possibly enough to act as a guide to sand it to shape.
I was having another think about it and another tool to cut thin plastic or score it would be the circle scribe (I think starshipmodeler did a review of it) and the makers site is here
http://www.circlescribe.co.uk/Another way to score the plastic in in circle would be to use one of those map compasses with two points rather then a pencil (many high end compasses are able to do this too).
Or the easiest way to cut thick plastic would be a circle cutter on a power drill, i'm not sure what sizes these come in but from the same place as the circle cutter in my original post they are £5 a set.
Hope this helps I think I really need more practice with writing on these forums so I don't come across so long winded :oops: