Put the raspberries in a large pan along with the lemon juice and simmer very gently for about 5 minutes, stirring. When the juice begins to flow from the fruit, bring to the boil, add the vanilla pod and boil gently for 5 minutes. Warm the sugar in a heatproof bowl in a low oven and add to the fruit stirring until it dissolves. Bring it back to the boil for 2 minutes. Take off the heat and spoon off any scum. Remove the vanilla pod. Leave to stand for 15 minutes, transfer to a sterilised jar and refrigerate.