A good bit of progress made, removed all of the ballast and chopped the shower tray out. The steel at the stern was a bit rusty, but it has been cleaned and treat with Fertan. The rest of the baseplate had surface rust, but the original paint was still mostly intact. This has been cleaned and a new coat of blacking applied. The Fertan hadn't cured enough before I came home, so that will be blacked next time I go.
The baseplate thickness is between 4.8mm and 5.9mm, I'm happy with that for a 38 year old boat.