You're declaring the XML namespace - but you're not actually using it (referring to it).
Change your code to:
XmlDocument xdoc = new XmlDocument();
xdoc.LoadXml(_LabourXMLInfo);
// define XML namespace manager based on the XmlDocument's NameTable
XmlNamespaceManager nsMgr = new XmlNamespaceManager(xdoc.NameTable);
nsMgr.AddNamespace("ns", "urn:smilu.com");
// add a "ns:" prefix to the XPath expression here!
string Number = xdoc.DocumentElement.SelectSingleNode("ns:Number", nsMgr).InnerText;