Autocompleteextender without webservice not working

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ASP.NET

Ajax

I've seen numerous posts and tips on CP and elsewhere that show people having successfully implemented the AjaxControlToolkit auto-complete without a webservice but I just can't get the page method to fire.

I think I've tried every approach out there, but to no avail. My code, at present, looks like this:

.aspx:

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" EnablePageMethods="true"  </asp:ToolkitScriptManager>

<asp:TextBox runat="server" ID="tbMatterReference" />
        <asp:AutoCompleteExtender ID="tbMatterReference_AutoCompleteExtender"
            runat="server" DelimiterCharacters="" Enabled="True"
            ServiceMethod="GetCompletionList" CompletionInterval="0" CompletionSetCount="1" ServicePath=""
            TargetControlID="tbMatterReference" UseContextKey="True"></asp:AutoCompleteExtender>

aspx.cs:

[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod]
public static string[] GetCompletionList(string prefixText, int count)
{
    MatterDetailsList matters = new MatterDetailsList();

    List<string> list = new List<string>();

    foreach (MatterDetails matter in matters.MatterDetails)
    {
        list.Add(matter.code);
    }

    return list.ToArray<string>();
}

There are no errors as such, it just doesn't do anything. Breakpoints in the code behind aren't hit and the TextBox just works as a vanilla TextBox.

I'm sure that I'm missing something blindingly obvious!