It would appear that Microsoft have sneaked in at least one update since I originally downloaded the SDK. The version of the dll I was using was 2.0.4330.0 but after discussion with Cindy Meister on an MSDN forum I discovered that there is (at least one) newer version.
I downloaded the SDK again (my original download was probably a couple of years ago) and the version downloaded (v2.0.5022.0) magically contained the required namespaces.
I hope this helps someone as it's taken a ridiculous amount of time to sort this out.