Place the fresh or frozen blueberries, whichever you're using, and the lemon juice in a liquidiser and whizz for a few minutes until the mixture is very smooth. Then push it through a sieve and set it aside.
To make the syrup, put the water and the sugar in a saucepan on a medium heat. Stir it just until the sugar has dissolved, then bring to the boil, boil for 1 minute, then remove the saucepan from the heat and set it aside.
Place the gelatine sheets in a bowl of cold water and leave them to sit for 3-4 minutes, until they have softened. Then squeeze out the excess water and add them to the hot syrup. Stir until the gelatine leaves dissolve, reheating the syrup if it has cooled down. Once the gelatine has been added, allow the mixture to cool until it is almost at room temperature. (If it's too hot, it will cook the blueberry and lemon mixture you add next, ruining the flavour.)
Add the blueberry and lemon mixture you set aside earlier to the gelatine syrup and stir together well, then divide the jelly between four glasses or cups and place them in the fridge to set - this should take about 3 to 4 hours.