So as per the instructions page, I tried:
sudo apt-key adv --keyserver
pgp.skewed.de --recv-key 612DEFB798507F25
However, this returns the following:
Executing: /tmp/tmp.UgWxLPVkr2/
gpg.1.sh --keyserver
pgp.skewed.de--recv-key
612DEFB798507F25
gpg: requesting key 98507F25 from hkp server
pgp.skewed.degpgkeys: key 612DEFB798507F25 can't be retrieved
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: keyserver helper general error
gpg: keyserver communications error: unknown pubkey algorithm
gpg: keyserver receive failed: unknown pubkey algorithm