Squeeze the juice from the lemons, reserving the pips. Put the juice into a preserving pan or large saucepan.
Scrape the pulp from the peel and put it onto a large square of muslin with the pips. Tie up the corners tightly and put it into the saucepan. Add the water.
Use a sharp knife to shred the lemon peel finely. Add the peel to the saucepan.
Put the saucepan on the heat and bring to the boil. Reduce the heat, then cover and simmer gently for 40 minutes to soften the peel, then remove the lid and simmer gently for a further 15 minutes, so that the liquid reduces by about one-third. Remove the muslin bag, squeezing it with a wooden spoon to push as much of the liquid as possible back into the saucepan.
Tip the Tate & Lyle Preserving Sugar into the saucepan and add the elderflower cordial. Stir over a low heat until the sugar has dissolved. Increase the heat and boil steadily for 20 minutes, taking care to keep an eye on the marmalade to make sure that it doesn’t boil too rapidly.
Meanwhile, sterilize 6 x 450g jars by washing them in hot soapy water, rinsing well, and placing them in a low oven at 150°C/Fan 130°C/Gas Mark 2 for 10 minutes.
Test the marmalade for setting point. To check, remove the saucepan from the heat, spoon a little marmalade onto a cold 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 until setting point is reached. (You may need to test several times, though be patient, as this testing is crucial to achieving the correct consistency).
Allow the marmalade to settle for 10 minutes, then pour it into the hot sterilized jars. Leave to cool, then seal and label. Store in a cool place for up to 1 year.
The marmalade will set upon cooling, so long as you have followed the instructions for testing the setting point, even though it looks quite runny when you first pour it into the jars.