Rinse the quinces, rubbing their skins well. Put into a saucepan with enough cold water to just cover them and add the lemon juice. Boil for 30 minutes. Take off the heat and leave the quinces in the liquid overnight.
Drain the fruit, keeping the liquid. Peel and core the quinces and then cut them into chunks. Put them in a large heavy-based pan and add the sugar. Measure the cooking liquid and top up with water until you have
Meanwhile, sterilize your jars for when you have a panful of hot jam ready to bottle. It is always best to use several small jars, rather than one or two big ones. Wash the jars and lids in hot soapy water, or in the dishwasher, and rinse well in hot water. Then put the jars (and the lids) on a baking tray and leave in a 120°C (250°F/Gas ½) oven for at least 20 minutes, or until you are ready to use them. (Don’t use a tea towel to dry them — they should dry thoroughly in the oven.)
Test that the jam is ready by dropping
When the jam is ready, remove it from the heat. If you like your jam smooth, mash it with a potato masher or purée with a hand-held blender. Spoon into the warm sterilized jars and close the lids tightly. Turn the jars upside down, cover with a tea towel and leave to completely cool (this creates a vacuum that can be seen on the lid). Turn upright and store in a cool dark place. The jam will keep for 10–12 months before it is opened. After opening, you need to keep it in the fridge.