Add half the strawberries to a preserving pan and crush them roughly with a potato masher.
Add the sugar, the remaining strawberries, the Champagne or wine and citric acid and heat gently for 10 minutes, stirring continuously, until the sugar has completely dissolved.
Increase the heat and boil rapidly for 5–10 minutes, testing at 5-minute intervals, until setting point is reached. Skim with a draining spoon or stir in butter if needed, then remove from the heat and leave the conserve to stand for 15 minutes to allow the fruit to settle.
Ladle into warm, dry jars. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and store in a cool, dark place.
To serve, spread over hot, buttered crumpets.