Wipe, peel, quarter and core the quinces. (Use the peel, and core if you like, to make about ½ pt / 280 ml pectin-rich extraction, which can be used to make jam or jelly with rhubarb.) Put the quinces in a saucepan with the water, sugar, cinnamon and half the butter. Bring to the boil, and simmer for 15–20 minutes. Drain, reserving the syrup for another use, such as sorbets. Return the fruit to the pan, and continue cooking, with the vinegar added, until tender. Sieve the fruit, and stir in the remaining butter. Cool, pot, and store in the refrigerator until the purée is needed.