Thank you all for the replies and the laughs !

    Will try the acid option to dissolve the scale. Prevention better than cure……my story…..

    When I suffered the fate full blockage my shipmates (ships engineer and chemical plant engineer) decided to employ the inflatable dinghy pump to pressurise the tank via the pump out point on deck having used a wooden bung to block the tank vent in the anchor locker. The question arose of how to de-pressurise as the blockage remained, the question was answered when the foot pump pipe shot out of the deck outlet, remember those ‘indoor fireworks’ !

    A length of clear braded plastic pipe, a wetsuit and safety line were the next weapons of choice. As I avoided the on deck muck spreading I drew the short straw and went over the side. The pipe was flexible enough to take the bend in the seacock and after some pushing and pulling the sniggers on deck revealed that it was working as a dark brown slick was seen developing around me, having my safety rope tied off on the midship cleat I had no means of escape until my so called shipmates gave me some slack, due to uncontrolled laughter this took some time !

    After a few beers and yet more laughter we decided another emergency back option was needed. When in the next marina I purchased a length of plastic shroud wire cover, following removal of the inspection hatch on top of the tank, there is just enough clearance under the deck head to feed the plastic tube (flexible too) into the tank and gently tap it up and down to break the scale up so that it cleared the exit point of the tank. Later in the trip it appeared that the tank was not doing the usual Niagara falls trick, we tried the shroud cover piping and it worked. All clear, no explosive turd, tank has been good as gold since.