Jquery - WebService issue

Jquery - WebService issue

Hi Guys and Gals

Im having some problems consuming a webservice.

The webservice works fine. I can access it and get a response.
However when i attempt to use it though the Jquery & Ajax it always returns and error. its probably something really small but as im faily new to this I cant see it.

Any help or suggestions would be great. Thanks in advance.

JQ & Ajax
$("#ddlAggregator").change(function()
          {       
               
                $.ajax({
                             async: true,
                             type: "POST",
                              url: "(servicename).asmx/methodname",
                              data: "{aggid:'1'}",
                              contentType: "application/json; charset=utf-8",
                              dataType: "json",
                              success: function(response)
                              {   
                              alert("asda");                                                                                                                                     
                              }           
                    });
          });


WebService
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ScriptService()]
public class dropListPopulate : System.Web.Services.WebService
{

    public dropListPopulate()
    {
    }


    [WebMethod(EnableSession = true)]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public ResponseType methodname(string aggid)
    {
        ResponseType resp = new ResponseType();

        //Some MySQL Query here
        MySqlDataReader reader = cmd.ExecuteReader();
        while (reader.Read())
        {
            resp.text.Add(reader["Brand"].ToString());
            resp.val.Add(int.Parse(reader["BrandID"].ToString()));
        }
        return resp;
    }
}
public class ResponseType
{
    public List<string> text = new List<string>();
    public List<int> val = new List<int>();
    public bool isValid = false;
}