OK, first take a very deep breath. Firstly UK Bullying have great guidence and get your child to talk to childline who are brillent. My son has just been through this. write to the school they have a legal duty to follow thier policy on bullying/anti bullying, behaviour and safeguarding, all of which you must read to know what they are capable of doing and should be doing. In your case the school failed to get back. If your child in suffering consider also discussing it with the Doctor (its always good to have a record there). My school lost all the bullying compliants so that Ofsted could not find them!.
You (and your child ) are in control. My child was convinced that his teachers were not doing any thing to help him and not believing him. so childline told him that the parents of the other child are responsible for his behaviour. My son (7) decided that he would write to the little so and so's parents and did so. I put a covering letter over the top, saying that I was sure that they would not expect such a letter, was my son in any way a catalyst etc. It was posted to them (recorded delivery), stating that childline was supporting my child and offering them a meeting to discuss the issues.
This way the reponsible adult has been informed, and they then are notified. Its the start of the evidence trail. you must also start a diary, noting incidents and getting the evidence together.
The school will be annoyed expecially if they do not find evidence of bullying, but you have a right to protect your child and a chain of evidence is useful, incase this continues.
Expect to be ignored by the head, and then if the other childs parents are part of the click group to be talked about behind your back.
I have found strong action, seems to have worked, but we are only two weeks in.
Hope it all works out