I've added an ASP.NET web-service, and an ASP.NET web application, and
added a web-reference using all the defaults.

My Default.aspx.cs can then use the service via:

using System;

namespace WebApplication1
{
   public partial class _Default : System.Web.UI.Page
   {
       protected void Page_Load(object sender, EventArgs e)
       {
           using (localhost.Service1 svc = new localhost.Service1())
           {
               this.Title = svc.HelloWorld();
           }
       }
   }
}