Malcolm,
You have fallen victim to the most common error when using functions... never nest your _expression_ evaluation brackets ${}
${directory::exists('${build.dir}')} becomes ${directory::exists(build.dir)}, and similar for your other expressions.
This is demonstrated in the fourth example of _expression_ syntax in the manual:
http://nant.sourceforge.net/release/latest/help/fundamentals/expressions.html#_expression_-syntax
Regards,
--
Troy
