This is my favorite Scottish TTS voice of all time!! I have worked with Cereproc to get it working with TalkBack. Initially the voice froze, so I reported this to Cereproc, and it was fixed! I am blind and must use synthetic speech every day, to complete every task on my phone. Not only am I immensely pleased with the voice, but I am quite pleased with Cereproc!! And, the reason the voice does not work with navigation anymore, is because Google does not allow us to use any voices, other than theirs, anymore, for navigation purposes.
Heather Scottish Text to Speech Voice at Google Play market analyse