OK, I've figured it out. I didn't spot that the tar contained a NEW .deb installation package. I was using the old one. This is partially my fault, and partially Navicore's -- the 770 upgrade tarball is corrupted and crashes all but command-line untaring tools, which reports several voice ogg files are missing. This confused me a little and I didn't spot the spread of new files... Usual story (Downloaded twice; same results each time. The voice files are for the svk, hun, fin, and dnk languages.)

I can see why they introduced an automated upgrade assistant. It's just a shame they only released this for Windows, and also a shame that it doesn't actually work