Chop the fruit into rough pieces, ensuring any bad bits are thrown into the compost bin. There's no need to peel or core the fruit, in fact you want as much of it as you can get. Quinces are quite tough, so use a sturdy knife.
Throw everything into a large lidded pot. Add just enough water to cover the fruit. Bring to the boil with the lid on and simmer for about 30-40 minutes or until the fruit is mushy.
Set a jelly bag or muslin lined sieve over a large glass or pottery bowl. Pour in the contents of the pan.
Once most of the juice has drained out, suspend the jelly bag or tied muslin cloth above the bowl and allow it to drip overnight or for a good few hours. You want to extract as much as the juice as possible, but don't be tempted to squeeze the bag.
The next day, sterilise your jars and throw the leftover pulp into the compost bin.
Measure out the juice and for every 600ml (1 pint) add 450g sugar and 50ml cider vinegar. I got 600ml exactly.
In a large pan, heat the mixture gently until the sugar has dissolved. Then bring it to the boil. Remove any scum - there will probably be a lot.
Continue to boil until the jelly reaches setting point. This could be anywhere between 10 and 20 minutes. Mine set after only 8 minutes.
Turn the heat off and leave a couple of minutes for the bubbles to subside. Stir in the chilli flakes, if using.
Pour or ladle into warm sterilised jars. Cover with wax discs, seal and label.
Store in a cool, dark place and refrigerate after opening.