Thanks to everyone who responded..

The solution turned out to be:
..
<xsl:when test="$sortParameter='name'" >

          <xsl:for-each select="query">
          <xsl:sort select="xalan:evaluate($sortParameter)"
order="ascending"/>

 Inside the loop I can get hold of the value simply again (just a test
in this case - I don't need the value here):
          <xsl:copy-of select="$sortParameter"/>
...

At all occurences of  '$sortParameter' I'm looking for its string value
ie 'name' (or 'date' or 'status').
Why is it that in the  <when> & <copy> tags I can call it simply but in
the sort tag it needs the xalan:evaluate() ?



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