I think it is the namespace causing an issue (note I don't fully get them so feel free to improve this answer)
doc.Load("http://localhost/xxxxx/Service1.svc/xxxxxx");
XmlNamespaceManager nsmgr = new XmlNamespaceManager(doc.NameTable);
nsmgr.AddNamespace("tu", @"http://tempuri.org/");
if (doc.SelectSingleNode("//tu:UpdateSalesHeaderResult", nsmgr).InnerText == "true")
{
}
else
{
}