I found a circumvention for my post of 7 Mar.  Most of my 'catalog' values were things like 'G  4' or 'BWV1063'.  And I thought the suggestion at http://www.dpawson.co.uk/xsl/sect2/N6461.html#d7476e270 might help, if I made sure that all catalog values contained a blank - then I could sort similarly to data-type="number" select="substring-after(.,' ')".  Alas, catalog values for the too-prolific Telemann look like '53 F: 1', meaning the first concerto for 3 instruments in the key of F.  I finally settled for:-
 
 <xsl:sort data-type="text" ... select="translate(detail/@catalog,
   ' ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
   '0ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789')"/>
 
Not especially elegant, but it seems to work.