Scrub the quinces and chop up roughly. No need to peel or core.
Place the fruit in a large saucepan and cover with the water. Bring to the boil and cook until the fruit is tender, mashing a little as it softens.
When cool enough strain the juice off through a jelly bag or piece of muslin, allowing the liquid to drip through. Do not squeeze or you'll get cloudy jelly. This is best done overnight.
Measure the juice into a large saucepan and to every 575ml add 450g sugar.
Bring to the boil and fast boil for 10 to 15 mins until setting point is reached. (A drop of the jelly onto a saucer which has been kept in the freezer will wrinkle if you push your finger gently across the top)
Remove from the heat and pour into warmed, sterilised jars that have been heated in a low oven for 10 minutes.