The Jenkins build for static was failing because Saxon was running out of memory. After a bit of experimentation with ANT_OPTS, which didn't work, I've now settled on this for the java tasks which run Saxon:

<java fork="true" classname="net.sf.saxon.Transform" classpath="../utilities/saxon9he.jar" failonerror="true">
            <jvmarg value="-Xmx1024m"/>
            <arg value="-s:xsl/xhtml_docs_master.xsl"/>
            <arg value="-xsl:xsl/xhtml_docs_master.xsl"/> 
            <arg value="--suppressXsltNamespaceCheck:on"/>
        </java>

This works; note that the task needs @fork="true" before the <jvmarg> element will take effect.