Could not download all repository indexes

I've recently installed Stretch (not upgraded from Jessie to Stretch) to a new workstation.

When I issue 'apt-get update', I receive the following error:

Could not download all repository indexes

The repository may no longer be available or could not be contacted because of network problems. If available an older version of the failed index will be used. Otherwise the repository will be ignored. Check your network connection and ensure the repository address in the preferences is correct.
GPG error: http://mirrors.kernel.org/debian stretch-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010The repository 'http://mirrors.kernel.org/debian stretch-updates InRelease' is not signed.Updating from such a repository can't be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.GPG error: http://mirrors.kernel.org/debian stretch Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B48AD6246925553 NO_PUBKEY 7638D0442B90D010 NO_PUBKEY CBF8D6FD518E17E1 NO_PUBKEY EF0F382A1A7B6500The repository 'http://mirrors.kernel.org/debian stretch Release' is not signed.Updating from such a repository can't be done securely, and is therefore disabled by default.See apt-secure(8) manpage for repository creation and user configuration details.The repository 'http://deb.debian.org/debian stretch/updates Release' does not have a Release file.

Please advise

Thanks

Cheers