Put the elderberries in a large bowl and add the cider vinegar and crush them lightly with a potato masher.
Pour into a large sealed glass container (I used a Le Parfait Jar) and leave to macerate for at least 2 days - 5 days is better if you have the time.
Attach the jelly bag to its stand and place it over a large bowl.
Pour the macerated elderberries and juice into the jelly bag and leave to strain for at least 2 hours or overnight.
Measure the strained liquid and pour into a preserving pan or large saucepan.
For every 600 ml (2 ¼ cups) of liquid add 450 g (2 ¼ cups) of granulated sugar.
Place over a low heat, stirring until the sugar is dissloved.
Bring slowly to the boil and boil for 10 minutes - 15 minutes if you want a syrupy vinegar.
Remove from the heat and skim off any scum.
Leave to cool before decanting into sterilised bottles or jars.
Store in a cool dry place away from direct sunlight - keeps for at least 12 months.