You can use frozen cherries for the jam, or make up the weight with a mixture of fresh and frozen. Cherries are low in pectin, so the lemon juice is important for aiding the setting process. Jars can be sterilised by running through the dishwasher or heating in the oven at about 120°C for 15 minutes.
For Sophie Gray's best cherry tips and tricks, go to In season with Food magazine: cherries.