<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0"
                >

<xsl:param name="uidselect="xxx"/>

<xsl:template match="/">
  <xsl:copy-of select="//site[uid=$uid]"/>
</xsl:template>

</xsl:stylesheet>

That is the complete stylesheet. using saxon (for example)

saxon original.xml
would pull out 
<site><uid>xxx</uid>....</site>

saxon original.xml uid=yyy
would pull out 
<site><uid>yyy</uid>....</site>


David

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