I've tried all sorts of things including predicates and variables, but I just can't seen to find a valid XPath expression that gives me the node set I need.

<xsl:template match="invoice">
   <xsl:variable name="record" select="invoice_rec/invoiceNumber"/>
   <xsl:apply-templates select="invoice_rec"/>
   <xsl:apply-templates
		select="../invoice_line[invoice_line_rec/invoiceNumber='$record']"/>
   ...
</xsl:template>