Put the raspberries in a pan with the sugar and vinegar. Mash over a low heat for 10 mins until smooth and syrupy.
Rub through a sieve into a clean pan.
To make sure you get all the pulp from the discarded seeds, tip them into a bowl of 300ml water then serve again.
Add the water to the pan, stir well and boil for 1 min.
Keep in the fridge until nice and cold, then enjoy in the garden on a sunny day.
If you wish to keep the cordial for later, pour into small sterilised bottles and seal. This will keep unopened for a few months.