What you are talking about is the defence of 'necessity', the leading precedent of which is F v West Berkshire Health Authority, in particular the reasoning of Lord Goff. Although on the surface the case seems to have no connection with what we are discussing, it establishes the precedent of the defence itself. You would need to show that there was no option other than the unwarranted laying on of hands to prevent the harm. In other words, if you manhandle the person because they are holding you up and you want to get the job done, rather than they are in imminent danger, then you have no defence. Them being thoroughly obnoxious and stupid is not, unfortunately, breaking any laws and they are not at that precise moment in any danger unless your subsequent actions, eg starting/continuing the cut or dropping timber which could be held for the time it took them to travel through the drop zone, puts them there. If, however, you have to physically stop them from entering the drop zone when there is wood on the way down (and no way of stopping it ie by not pushing it off in the 1st place) then you can legitimately say that you did what you did to prevent them from harm. You can be certain that your system of work will be closely examined to make sure that the situation could not have been avoided any other way, including better planning beforehand. Your actions will need to be seen to be an absolute last resort when all of your well laid plans were overridden by something which could not be anticipated.