Wash a bottle with a vinegar-proof screw-top or stopper before putting them into an oven at 130°C/275°F/Gas mark 1. Alternatively, put both the bottle & screw-top/stopper in a large saucepan and fill with enough water to cover the bottle completely. Bring to a boil and simmer for at least 15 minutes

Rinse the berries in a large sieve or colander and remove any stalks and leaves

Put them in a saucepan with the vinegar and water

Bring the mixture to a boil, lower the heat and simmer for 30 minutes, by which time the berries will have turned a dull brown and their skins will have split to reveal their yellow flesh

Tip into a sieve over a clean pan and rub the fruit through with a spoon, leaving the skins and pips behind

Add the sugar to the purée in the pan and heat gently, stirring until it dissolves

Bring to a boil and simmer for 5 minutes, stirring often to ensure it doesn't catch

Funnel the hot sauce into the hot, clean bottle and seal straight away