I've had the boat battery on charge for a few days now and its just about struggled to 13.3 volts. It looks like its been disconnected from the solar panel for a couple of weeks and the bad news is it may not recover fully.
I might have to bite the bullet and buy a new battery. :-(
I may have to use my contacts and see how cheap I can get a leisure battery. But not before I fit a solar charging regulator.