2 lemons, juiced
About 500g granulated sugar
Gently wash the fruit by dipping it in cold water. Pull the berries off their stems and place in a saucepan. Crush well to release plenty of juice, then add the water. Bring to the boil, cover and simmer for about 15 minutes or until tender.
Pour into a jelly bag and leave to drip overnight. Discard the berries and measure the liquid. Weigh out the sugar (455g of sugar to every 565ml of juice).
Put the elderberry juice, lemon juice and sugar in a preserving pan and clip a sugar thermometer onto the side. Heat gently, stirring occasionally, until the sugar has dissolved, then increase the heat and bring to the boil. Boil vigorously, stirring regularly, until it reaches setting point (106˚C) on the thermometer. Meanwhile, sterilise your jam jars by washing them in hot, soapy water, then rinsing in hot water. Place in a warm oven to dry.
Remove the jelly from the heat as soon as it reaches setting point. Skim off any scum and pour the jam into the jars. Tightly cover the jars with a damp disc of waxed paper (wax side down). The discs will contract as they dry, forming an airtight cover. Label and store in a dark, cool place.
Mark Price is managing dir-ector of Waitrose (firstname.lastname@example.org)