It might be enough just to do:
<xsl:variable name="x">
<xsl:choose>
<xsl:when test="string-length(@number) > 10">35</xsl:when>
<xsl:otherwise>25</xsl:otherwise>
</xsl:choose>
</xsl:variable>
<fo:list-block provisional-distance-between-starts="{$x}mm">
--
http://www.dcarlisle.demon.co.uk/matthew
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