Thanks for the suggestion Michal. Casting to string would of course be simple but the SP is widely used elsewhere and I don't want to be changing an output data type because that may break other interfaces.

On the face of it it seems that Microsoft don't support GUIDs across their technology stack which just beggars belief. As a developer I want to spend my time solving business problems but with Microsoft I seem to spend most of my time trying to code my way around gotchas in their implementation.