sudo apt-get update produces:
Err:8 http://ppa.launchpad.net/dansmith/chirp-snapshots/ubuntu zesty/main amd64 Packages
  404  Not Found

and - 
W: The repository 'http://ppa.launchpad.net/dansmith/chirp-snapshots/ubuntu zesty Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/dansmith/chirp-snapshots/ubuntu/dists/zesty/main/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

Thanks,

Simon R.