Directions

1. Tip the sugar into a heavy preserving pan or flameproof dish with the plums, lemon juice and half the raspberries. Heat until the sugar has dissolved, then bring to the boil and simmer for a few mins until the plums are tender. Add the rest of the raspberries to the pan.
2. Place a jam thermometer in the pan, bring to the boil, then cook over a high heat until the temperature reaches 104C. Alternatively, put a small plate in the fridge or freezer until really cold. Once the jam has been boiling for 10 mins and looks thick and syrupy, turn off the heat and pour a spoonful onto the plate. The jam is ready when it wrinkles as you push it with your finger; be careful not to burn yourself. If the jam is not ready, boil for another 3 mins, then repeat test as above, until it is ready.
3. Allow to cool for about 5 mins. Skim the surface of any scum and pack into warm sterilised jars (see tip below). Cover with wax discs and lids.Will keep in the fridge or a cold larder for up to 6 months.