WCF REST Web Service and HTTP Headers

I've created a REST web service that returns a simply id passed in xml format.

However, as part of the xml returned, I want to include custom headers. I want to be able to read the data in the request headers and return some of them back in the response.

For example, if the request includes Header1 and Header2, I want to return Header1 and Header2 as part of the response, along with a new header, Header3.

I'm struggling to work out where and how to do this so any help would be appreciated.

My code:

RestServiceImpl.svc.cs:

namespace RestService
{

public class RestServiceImpl : IRestServiceImpl
{
    #region iRestService Members

    public string XMLData(string id)
    {
        return "You requested product " + id;
    }

    #endregion
}
}

IRestServiceImpl.cs:

namespace RestService
{
[ServiceContract]
public interface IRestServiceImpl
{
    [OperationContract]
    [WebInvoke(Method = "GET",
        ResponseFormat = WebMessageFormat.Xml,
        BodyStyle = WebMessageBodyStyle.Wrapped,
        UriTemplate = "xml/{id}")]
    string XMLData(string id);

}
}