[tor-relays] Tor Install Error

While using apt-get to update my tor relay to a 'recommended' version I keep encountering this error:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://deb.torproject.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 74A941BA219EC810

W: Failed to fetch http://deb.torproject.org/torproject.org/dists/trusty/InRelease

I've read several approaches to remedying this error, however I'm wondering what would be the 'most widely accepted as correct' method of dealing with the error?

Thanks