Thanks very much, that works a treat with one minor change.
I amended the line
<xsl:if test="not(Plot = preceding-sibling::*/Plot)">
<xsl:if test="not(Plot = preceding-sibling::OrderLine/Plot)">
Not sure if this is because I didn't post my complete source XML or some other reason.
Anyhow, it works now, I can enjoy the rest of my Friday afternoon now