Put the raspberries and lemon juice in a large heavy-based saucepan. Gently stir over low heat for 10 minutes, or until the raspberries have softened. Warm the sugar by spreading it in a large baking dish and heating in a 120°C (235°F/Gas ½) oven for 10 minutes, stirring occasionally.
Add the sugar to the saucepan and stir, without boiling, for 5