Chop half the quinces, including the peel and cores, and put them into a large stainless steel stockpot with the water and bring to a boil over a low heat. Simmer for 30 minutes or until the fruit is very soft.
Tip the contents of the stockpot into a prepared jelly bag, or layers of muslin, that is suspended over a bucket or large bowl and let it drain for 12 hour