Mash the banana, using a wooden spoon oR fork. Melt the butter slowly in a heavy based pan, then add the sugar, bananas, lemon rind and juice. Cook for 10 minutes. Beat the eggs with a wooden spoon, then gradually beat in 3 tablespoons of the banana mixture, (doing this; perevents the eggs from curdling.). Pour onto the remaining banana mixture, stirring constantly, until the mixture thickens and is thick enough to coat the back of a spoon; but do not boil. Pour into hot sterilized jars and seal.