~/.ssh/config

Do the rss key thing on both citadel and login.hpc.cam.ac.uk

From outside campus:

Host camhpc darwin login.hpc.cam.ac.uk
  HostName login.hpc.cam.ac.uk
  User nsa12
  ProxyCommand ssh nsa12@citadel.ch.cam.ac.uk nc %h %p 2> /dev/null
  HostKeyAlias login.hpc.cam.ac.uk