OK, I have it. Using MidiMock I had to set the destination to my Network Session. My own app I had to direct its events to the network session using the Live Routings in Audio MIDI setup.
It's a little fiddly, I have to set the route avery time I fire up the app, I don't suppose there's a way to make the routing stick, or just have my app decide where to send events? Long shot, maybe.