I've placed xercesImpl.jar into /var/lib/tomcat5.5/webapps/queuemetrics-1.4.7/WEB-INF/lib , this gets rid of the xml error.
However, after restarting tomcat I get the following (and nothing is logged to /var/log/tomcat5.5/catalina)
HTTP Status 404 -
type Status report
message
description The requested resource () is not available.
Apache Tomcat/5.5