Rinse the berries thoroughly. Discard any berries showing the slightest sign of mould.
Put the remaining berries in a pot along with the water and boil for about 10 min. until the berries start to turn tender. Add the sugar. If you’re using vanilla, also add the pod and seeds (which have been has scraped out of the halved pod). Boil for an additional 10 min. If you don’t want whole berries in your jam, you can give it a gentle whiz with a hand blender. Add lemon juice to taste.
Test the consistency of the jam using a spoon or plate from the freezer. Continue to cook until you have the desired consistency. If you don’t want to cook it that long, you can add some powdered pectin, which is available in supermarkets. It will help your jam to set. Follow the instructions on the package.
Pour the jam into clean, scalded jars that have been sterilised with sodium benzoate or vodka. Place the lid on immediately.