Chop fruit up roughly and put into pan, almost covered with water, and add the acid. Bring to boil and simmer gently until the fruit is soft and broken down. Press it through a sieve and weigh the pulp. Return to pan with the sugar. Dissolve the sugar, bring to the boil again and boil gently for about 1 hour, until creamy, stirring constantly. Pot and cover in usual way.