On our boat the plywood base to which the seat attaches is held down to the woodwork over the fridge by brass screws. The seat and plywood can be lifted off as one. I imagine the screws holding the seat to the base would be visible once you've removed the whole seat and base, but I can't say that I've ever looked. I just needed to remove the seat to get at the bulkhead while enclosing the wheelhouse.
On our 27 the seat is secured by 6 hex head screws accessible from the locker beneath. You need to remove the sliding table first , the stop has 3 small brass screws. easy but you need to stand on your head in the locker! Paul
Thank you. Job done! Remove fridge (4 screws), , remove sliding table (3 screws), remove seat (5 hex head screws). Take seat to reupholster (£80.00) and replace in reverse order. After over thirty years the seat cushion was badly worn on the rt hand corner for obvious reasons. The joy of a wooden interior that can be disassembled!