1kg sugar approx
Juice of 1 lemon (if using ripe quince)
When making a jelly, I work on the principle of simmering 500g of fruit with the same amount of water ie 500ml. So, with the quince, I quartered them and added the fruits, seeds, core, peel and everything to the pan and simmered with a lid on for half an hour until the fruit was soft and pulpy.
Strain the fruit pulp through a jelly bag overnight. Weigh the drained fruit pulp the next day and add the same volume of water to the pan and simmer again for 20 minutes before straining again for a couple of hours.
When all the fruit juice has drained through, measure the liquid into a pan with a measuring jug and add 500g of sugar for every 500ml of liquid and the lemon juice if using ripe fruit. Bring to the boil and then leave on a rolling boil for 10 minutes taking care it doesn't boil over.
After 10 minutes check for a set on a chilled plate. When a spoonful of cold jelly wrinkles when you push your finger through it, it is done. Take the jelly of the heat. Jelly does seem to create a scum very easily so pour gently through a sieve into sterilised jars and seal.