Place the frozen blueberries in a large saucepan and add the Jam sugar and then add the lemon juice. Place the squeezed out lemon halves and the leaves from the teabags into the centre of a square of muslin. Tightly tie with string and add to the pan. Slowly bring to the boil, stirring occasionally until the sugar has dissolved.
Cook over a medium heat to boil gently for 10 minutes, then test for setting point. To check, remove the saucepan from the heat, spoon a little onto a refrigerated plate and leave for 2 minutes – it should wrinkle softly when your finger is pushed over the surface. If this point has not been reached, return the saucepan to the heat and continue to boil for another 2-3 minutes. Test as before. (You may need to test several times, though be patient, as this testing is crucial to achieve the correct consistency).
Turn off the heat and allow the jam to settle for 10 minutes. Meanwhile, sterilize 5 x 400g clean jam jars by placing them in a low oven at 150°C, fan 130°C, Gas Mark 2 for 10 minutes. Stir the jam, then pour it into the hot sterilized jars. Seal with the lids. Leave to cool, then label. Store in a cool place for up to 1 year.
Cook’s tip: If you’d rather not have the flavour of Earl Grey tea, simply leave out the tea bags, or use lemon and ginger flavoured tea as an alternative.