By writing a <SubConcept> node in this form, the output xml file ensures that a <SubConcept> node definitely exists. However, I want the output xml file to have a <SubConcept> node only if

well stick an xsl:if around it that does that test then.

One way I was thinking of coming round this problem is to declare a global variable and then assign it a value locally

XSLT doesn't have separate declaration and binding of variables.
If you have declared a variable it already has a value (and it you can't
give it another one.)

Regards
Rahil