Replaced one on my hillyard. top nut came undone easy enough. On good advice from John Lilley i made up a knocker inner from an old spark plug spanner. fitted directly over the rod so i could give it a good smack. and a good smack it needed. i was concerned that it might be broken in the middle or corroded in the middle so i got one of the kids to lie underneath and feel for movement ( their mom wasnt there). eventually it started to move. not much room under the keel so as it came out i cut it into pieces. it wasnt technically difficult but it did require some effort.
Sorry, I have been told in the past about being too technical. It's my university education what does it.
By knocker inner I mean something that sits over the rod nice and snug and up out of the floors. Something to aim at so you can really smack it with a decent hammer. It's difficult on my boat to get a good contact and you don't want to miss and hit wood or even hit the bolt at an angle.
Have fun.