Ajax call to .asmx page method failing with invalid System.InvalidOperationException: Missing parameter

I am trying to use a jquery autocomplete with an ajax call to a webservice method the call works fine on localhost but when published to the dev server it fails everytime and using google chrome to debug i get the following error:

System.InvalidOperationException: Missing parameter: sLookUP. at System.Web.Services.Protocols.ValueCollectionParameterReader.Read(NameValueCollection collection) at System.Web.Services.Protocols.HttpServerProtocol.ReadParameters() at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()

I am using ASP.NET3.5 with IIS 7.5 where as the dev machine is IIS 6 with ASP.NET 2.0

My ajax call is as follows:

        $("#<%=txtSearch.ClientID %>").autocomplete({
        source: function(request, response) {
                $.ajax({
                url: "http://ift.tt/1GGf9sg",
                data: "{ 'sLookUP': '" + request.term + "' }",
                dataType: "json",
                type: "POST",
                contentType: "application/json; charset=utf-8",
                success: function(data) {

My webmethod in Address.asmx :

[WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public String[] ShowAddress(string sLookUP)
    {
        List<string> lstAddresses = new List<string>();

        DataTable dtAddresses;
        Address Addr = new Address();
        dtAddresses = Addr.GetLookupAddresses(sLookUP);


        foreach (DataRow row in dtAddresses.Rows)
        {
            lstAddresses.Add(string.Format("{0}-{1}-{2}-{3}-{4}-{5}-{6}-{7}-{8}", row["OrderAddress_name"], row["OrderAddress_id"], row["OrderAddress_1"], row["OrderAddress_2"], row["OrderAddress_town"], row["OrderAddress_county"], row["OrderAddress_postcode"], row["OrderAddress_fulladdress"],row["OrderAddress_ClusterID"]));
        }
        return lstAddresses.ToArray();
    }

web.config settings (there's been suggestions on other questions these should help:

<webServices>

      <protocols>
        <add name="HttpSoap"/>
        <add name="HttpPost"/>
        <add name="HttpGet"/>
        <add name="Documentation"/>
      </protocols>
    </webServices>




<system.webServer>
    <handlers>
      <add name="ScriptHandlerFactory" verb="*" path="*.asmx" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
      <add name="ScriptHandlerFactoryAppServices" verb="*" path="*_AppService.axd" preCondition="integratedMode" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    </handlers>
  </system.webServer>

Any suggestion welcome I've tried so many things but as it works on my machine i'm thinking its an iis/web.config setting I've missed or written incorrectly because the dev machine has a different setup. Thanks