Every so often, you just make a mistake. After a lot of chutney-making, which involves a very long boil until a spoon drawn across the base of the pan leaves a trail, I lost my touch with jam. After dad asked me to use up some plums, I created the most ridiculously toast-ripping plum toffee in a jar to be imagined. Luckily, a little troubleshooting on the internet found a solution.
You'll need two saucepans, the offending jam, a butter knife and some water. Half-fill one of the saucepans with water, and put the jam-jars inside, with the lids off. Slowly bring the water to a simmer, and edge the butterknife down the edge of the blocks of jam inside until they are coming away from the sides. If need be, clean and heat the knife in the simmering water as you go.
Upend the jam jars into the other saucepan, and prise the jam out. Pour some of the hot water into this pan, and mush up the blocks of jam. Now bring the watered-down mix to the boil again until you have acquired a real jam set. Test this by dropping a blob into a cup of icy water, or onto an icy plate.