Mark the decks are mahogany faced marine ply which is then screwed down on the four bearers rather than being glassed over like most ribs. I suppose over a period of 20 years water will have an effect but at least they are easy to replace by simply useing the existing piece as a template.
I am hoping Ian is going to rip the whole lot up and fit a cabin. If it works I might follow his example
Shame its half way through the season though, perhaps just replace the 4 x 4' deck piece until the winter. With a bit of luck the console will only need lifting a couple of inches to access the screws underneath it.