I have just had this problem sorted on my 1979 Nauticat 33 Mk11. Over a number of years water had been leaking through the deck glands for all the mainmast electrics, radio gps etc. the water ran down the cables and through the main thwartships beam just above the sliding door. The beam was rotten and so the mast step and surrounding deck area had subsided. Head lining removed along with adjacent panels and bulkhead to get access to replace main beam. Not noticed earlier through various surveys as the headlining hid the destruction that was taking place. Costly repair but only solution £2.5 K. King post not damaged but main beam was almost non existant.
Hope that helps