I noticed a few cracks at the foot of the mast, on investigating found years of water ingress and a collapsed mahogany block! Glad it had not started to crush the surrounding deck sandwich. I have just completed the repair having cut out and repaired with epoxy and woven mat. Stainless plate strengthens the job and tidies up the area.
A reasonably easy job but it always feels worrying taking a cutter to the deck!