Moving V2.4 to Mainsail

 Just starting the migration to Mainsail for my 2.4 (serialised as V2.924)... and coming up against the apt-get update “the following signatures couldn’t be verified because the public key is not available”.... the following command seems to work. (source)

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys [your key]