I agree with lots of good advice here. You ahve had some excellent posters saying very wise things do sift through and not what is right for your child.
AspieUnicorn although I agree This isn't good enough on the part of the school. If I were you I would write a letter to the head explaining why your son needs to be moved to another class. Written correspondence cannot be ignored. You son has the right to feel safe at school. I would say why does the 'victim' need to moved. The bully should be moved. Also it is possible to have a policy at a school where X child is not allowed to be within so many meters of Z child. This means the child who is offending needs to keep away from the other child, not the other way round. This means they can't be in the same class or study group together and the bully needs to stay away from the child he has been mercilessly bullying for so long. If this proves too hard for the bully they should be required to leave the school.
pinkyredrose re Police? Are you mad ?? I am not sure it is mad to consider someone trying to strangle you and dragging you by your feet through the mud as something that the police might be concerned with.
I don't think (sadly) the police will be interested or take action but I would consider threatening it. If the police were told someone (no more info) had been the victim of incidents of strangling, pulling trousers and pants down, hitting, pushing and stamping.... covered in mud and with scratches all over ... stomach and unhappy.... pushed over then dragged by ... legs across the field don't you think they would consider that a criminal offence? 
Suddenly, when the victim and the perpetrator are children some might think it is not so serious. I think it may well be just as serious, this little bully has a whole time ahead of them to continue their vile behaviour. Maybe the school should take it seriously.
expatinscotland your example of a child being thrown down stairs is utterly appalling. Sadly, I have long felt unless schools are sued this culture of behaviour will not change. If someone did something like that to me at work and my work place did nothing, I would sue the work place and expect them to deal with the bully (after i called the police).
If the child is below the age of criminal responsibility then surely there is some other way of safeguarding the Op's child? The child may have special needs, may be the victim of bullying or abuse in their own home and may be acting out as a way to get the attention they need. This does not excuse the behaviour. As pthers have said if the school fails to address this it has failed both the Op's child and the bully! At the very least there might be some sort of community police who could be involved, could there?
I agree with ImperialBlether in that I'd want an urgent appointment with the headteacher and wouldn't send my son to school until he/she took action. (but as an aside I'm really shocked that this boy has been strangling others. I've never heard of a child doing that to another child. I know of at least two instances at our school where hands were around the child's throat, one was my child.)
My gut feeling is to go with ShamefulPlaceMarker advice and I'd be pulling my son out of the school tbh. I know it's unfair, as it should be the other child who goes. But it doesn't seem like that will ever happen.
But I would also start some sort of complaint procedure, through Ofsted or whatever, about the way the multiple cases of bullying and assault have been handled. At the very least I would threaten to take my child out and threaten to start a complaint procedure. Sadly, Ofsted and all their bollocks have probably made it so that schools where kids do academically well get good ratings and schools that care about their pupils' well being do not (sorry my own feelings coming through!).
Idefix I am so sorry for your ds's experiences. And so angry with you that the school failed him so badly.
Runningoutofnamesagain please make a list of all the options mentioned here. Choose which ones you would like to try and make sure you have your list of comments and 'evidence' e.g. previous assaults and the lack of action, with you see the head tomorrow. It is quite likely in the heat of the moment to forget one thing and you want to tell the full story. I think you also need to be 'smart' in how you reveal your information. Do not throw it all at them at once, then they can come back with some smart and stupid replies and you are left just reiterating.
Maybe start with the incident at hand and then hear out their weak bleating comments, then follow up with things from the past and how in each incident they have failed. Ask for reassurances in writing about what they will do. Then decide if you wish to send your son back to this place or move him or what you wish to do.
Are you parenting alone? Do you have a partner or good friend or relative who can come with you and another to look after your son while you go.
Good luck.