Add the quinces to a large pan and pour over the measurement water. Cover and bring to the boil, then reduce the heat and simmer for 1 hour, until very soft.
Purée the quinces and their liquid in batches in a food processor or blender, then press through a sieve into a large bowl. Discard the seeds, skin and cores. Weigh the purée. Wash and dry the pan and brush with sunflower oil. Pour the purée back into the pan, add 500 g (1 lb) sugar for every 500 g (1 lb) purée, and cook over a low heat, stirring from time to time, until the sugar has dissolved.
Cook, uncovered, over a medium heat for 45–60 minutes, stirring more frequently towards the end of cooking, until the mixture makes large bubbles, has darkened slightly and it is so thick that the wooden spoon leaves a line across the base of the pan when drawn through the mixture. Skim with° a draining spoon or stir in the butter, if needed.
Ladle quickly into warm, wide-necked jars with tight-fitting lids (square if possible), the insides of which have been lightly brushed with sunflower oil, filling to the very top. Cover with screw-top lids, label and leave to cool.
To serve, loosen the quince cheese with a round-bladed knife, turn out and slice. Serve with bread and cheese.