+2  A: 

Add the following directive to your app.config

<startup useLegacyV2RuntimeActivationPolicy="true">
  <supportedRuntime version="v4.0"/>
</startup>

This should fix your mixed mode problem.

Henrik P. Hessel
thanks got it (did initially put it in the wrong app.config, doh) - pit the framework couldn't just work out itself when such mixed mode compatibility is required
Greg