Meanwhile, place the berries, lemon juice, vinegar and 1/4 cup water in a large pan. Add the sugar, stirring over low heat until it fully dissolves. Increase heat until mixture is at a slow boil. When scum forms on the surface, remove with a slotted spoon. After 20 minutes, test to see if it has reached a set - place a spoonful onto a chilled plate and push mixture with your finger. If it wrinkles and a skin forms, then it's ready; if not, wash plate and return to freezer. Cook jam 10 minutes, then test again. Continue until jam reaches a set. Pour into sterilised jars.
