I think you are supposed to be allowed to move if 5m back from the mark, but sometimes the umpire still tells the player on the mark to stand (not 5m away maybe?). But if they are not 5m away then surely they should then be able to move to the actual mark, although this is very inconsistently applied.
The other issue is when a player runs to stand the mark and the umpire is confused as to whether they are going outside 5m or trying to get to the actual mark, what often happens is that the mark gets moved forward a couple of metres to the advantage of the player who has taken a mark or received a free.
Absolute mess really.
DS