Cover with the water and boil until the water is reduced to about half the quantity.
Pour into a heat-proof bowl, cover with cling wrap.
Allow to stand overnight on the bench.
In the morning strain through a sieve and measure the liquid. Throw away the lumps.
Sterilise the jars and keep them warm.
Place the liquid into a saucepan with the sugar. Heat over medium heat, stirring until the sugar has dissolved.
Bring to the boil, stirring frequently so the mixture doesn't burn or stick to the bottom of the pot. If you have any froth appear on the top of the mixture, scoop off with a spoon.
If you have a candy thermometer, it needs to read 105°C (220°F), though you will need to keep a close eye on the mixture when it starts getting around this temperature.
If you don’t have a thermometer, you can use the jam / cold plate test. Before starting your jam, simply place 3 or 4 small plates in the freezer. When you think the jam may be ready, place a small teaspoon amount of jam onto one of the cold plates from the freezer and let it sit for about 30 seconds. Then run your finger through the middle of the blob of jam. If the line starts to fill back over, it means the jam is too runny and is not ready yet, so keep cooking and repeat again in about five minutes. If the line remains, then the jam has reached setting point and is ready.
Fill your sterilised jars with the jam and seal them while they are still warm.