Quarter the strawberries into a glass or stainless bowl.
Sprinkle 800g sugar evenly over the strawberries and gently fold in to get an even coating. Cover the bowl and leave overnight to soak thoroughly.
Gently tip the strawberries into a large pan and stir gently.
Mix the Mamade and remaining 500g sugar together and gently add to the strawberries. Mixing well, gently heat the pan until the sugar is completely dissolved.
Bring to the boil and turn down the heat slightly and stirring occasionally to prevent burning on the bottom of the pan, keep on a full simmer for about 20 minutes. Gradually bring the pan back to a rolling boil stirring occasionally and test for a set after 10 minutes, using a chilled saucer to test a sample teaspoonful. If it sets after a couple of minutes, then remove from the heat.The marmalade should reach setting point of 106℃.
Leave the marmalade to cool and settle so the strawberries don't rise to the top when potted. Remove any scum from the surface with a large metal spoon and stir well to combine the fruits.
Pour the marmalade into clean, warm sterilised jars and cover with waxed discs then lid tightly. Leave the jars upright to set, and store in a cool place.
Notes
For a luxury flavour which makes a lovely gift, add 2 tablespoons of Cointreau or Whisky to the pan halfway through the boiling process.