1. Put the raspberries in a large saucepan and heat gently until they soften and the juices start to run. Press the raspberries with the edge of a wooden spoon to break them up, then stir in the sugar and lemon zest and juice. Heat gently until the sugar dissolves, stirring frequently.
2 Bring the mixture to the boil and boil for about 5-6 minutes until setting point is reached. To test, place a teaspoon of the jam on a small plate and chill for 2 minutes. The surface should wrinkle when pushed with a finger. If necessary, boil the jam a little longer. If the surface of the jam becomes foamy, skim it off with a slotted spoon.
3. Ladle the jam into steralised jars, cover with waxed discs and lids. Label and store for up to 12 months.