Thanks for the replies. I had to read them a few times before I was clear on everything but they explain everything very well. Much appreciated!
I did manage to get it apart (except the fixed cup) using a gear puller from Auto Zone to remove the crank arms, the hook spanner I had, which I think came from a Raleigh tool kit and a big adjustable wrench.
I understand replacing the unit with a modern replacement makes sense but everything inside looks usable so I'll just clean it up, re-grease and reassemble.