I sent email to readers letters in Yachting Monthly egarding smelly toilet, and received interesting reply, aparently smell is caused by sea life in the inlet pipe which die when seacock turned off and left for several days, the cure is to dissconect the inlet pipe from the seacock, insert a tee piece into the oulet pipe from the basin just above the seacock, and connect the toilet inlet pipe to the tee piece. To operate the toilet flush you need to fit the basin plug, and when you leave the boat fill the basin with water from the tap,turn off the seacock and then flush the toilet using the water in the basin, this removes any sealife from the inlet pipe to the toilet and leaves it full of fresh water, then when you return and use the toilet there is no sea life to die in the pipe