I got this working eventually.
try{
   XPath expression = new DOMXPath("//root/nodex");
   result=(Element) expression.selectSingleNode(xmlDocument);

   //This worked from the outset
   result.setAttribute("nxattr", "newatrrvalue");

   //This didn't work. The output showed the node with an updated attribute
value, but the text unchanged
   //result.setNodeValue("My New Data");

   //This, however, worked just fine
   result.getFirstChild.setNodeValue("My New Data");

   System.out.println(result.toString());
}
catch(JaxenException e){e.printStackTrace;}
catch(DOMException e){e.printStackTrace;}

So when selecting a node, it appears that the Nodes text value is a child of
the node, although the attribute is not.

Gerorge