Use of extractValue

Hopefully a very simple question for all you XML gurus out there.  I have just started using XML on our Oracle database (version 11.1) and have been reading through the documentation and have set up the following as a simple test:

set long 10000

create table test_xmltype_tab of xmltype;

insert into test_xmltype_tab values
(xmltype('<?xml version="1.0" encoding="UTF-8"?>
<ns1:activateWebUserResponse xmlns:ns1="http://fakecompany.co.uk/service/webuser/v6">
  <siAudTransactionId xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true" />
  <valid>true</valid>
  <webUser>
    <username>myusername</username>
    <emailAddress>donotreply@fakecompany.co.uk</emailAddress>
    <activatedInd>true</activatedInd>
    <webUserType>Client</webUserType>