Put two small, sterilised saucers in the freezer – you will use these to test the setting point of the jam. You can sterilise the saucers by washing them in hot soapy liquid, rinsing and placing in an oven at 180C/160C fan/gas 4 for 5 mins. Remove carefully and allow to cool.
Wash and drain the fruit, cut the apple into 2-inch pieces and tip into a large, heavy-based saucepan with 200ml water and the lemon juice. Bring to the boil, then reduce the heat and simmer with a lid on for 10-15 mins until the fruit is soft. Stir occasionally using the back of a wooden spoon to help break down the fruit.
Remove from the heat and allow to cool a little. Place the fruit in a muslin lined sieve over a large mixing bowl and allow the juice to drip through. Discard the contents of your sieve, retaining your muslin to use another time.
Weigh the remaining liquid and return to the saucepan. Weigh three quarters of sugar to your total liquid amount (for example, if you have 400g of liquid, add 300g of sugar). Add the sugar to the berry liquid and stir over a low medium heat until completely dissolved.
Turn up the heat and bring the fruit to the boil, stirring occasionally, and allow to boil hard for about 8 mins. If using a thermometer, you’re looking to reach 105C. If not, check the setting point of the liquid after 8 mins by removing a saucer from the freezer and adding a teaspoon of the berry liquid to the plate. Allow to sit for a minute. Push your finger through the liquid – if it starts to wrinkle the jelly is ready. If not, return to the boil and try again after another minute.
Pour into sterilised jars and allow to cool completely. Store in a cool dark place.