I have managed to get all three methods to Launch the Purge Unused command: SendWait, AutomationElement & PostCommand. Unfortunately is seems that once the purge dialog appears it squashes any further response from the api command. My goal of Purging multiple models has been hindered :-(
It seems that the only way to achieve this would be to drive the opening, purging, saving and closing each model entirely from an external application. I'm not sure it is worth it.
Kind Regards
David