I thought I would give some feedback, it seems that I made two mistakes...

1) I typed the incorrect version number, so this was why it could not find
the explicitly named jar file.

2) even after fixing the jar name this syntax did not work...

<taskdef resource="net/sf/antcontrib/antlib.xml">
<classpath>
  <pathelement location="/lib/ant/ant-contrib-1.0b3.jar"/>
</classpath>
</taskdef>

(yes I did correct the jar name by this point!)

but this did work...

<taskdef resource="net/sf/antcontrib/antlib.xml">
<classpath>
  <fileset dir="lib/ant" includes="*.jar"/>
</classpath>
</taskdef>

Does anyone know what was wrong with the first one?

Cheers,
Rob.