Put the blackberries into a large mixing bowl and crush lightly with a fork.

Pour over the vinegar, stirring to combine into the blackberry pulp.

Transfer to an airtight container, cover and leave to steep for three days in the refrigerator, stirring occasionally.

Place a large piece of muslin, double the size of a large mixing bowl (with a spout if possible as it will make life easier later) and drape on the inside of the mixing bowl so it overhangs on the edges.

Pour the blackberry and vinegar mixture into the mixing bowl and tie up the corners of the muslin so the mixture is secure inside.

Suspend the bag of fruity vinegar over the bowl, using kitchen twine. Hang it from the handle of an upper kitchen unit perhaps.

Leave the vinegar mixture to strain into the bowl for a couple of hours.

Measure the liquid, pour into a saucepan and add 50g of sugar for every 500g of fruit vinegar.

Bring to a gentle simmer, on a low heat, stirring until the sugar dissolves.

Remove from the heat and pour into sterilised jars and seal. Use within 6 months.