Tip the sugar into a large saucepan and add the lemon juice and 100ml water. Bring to a simmer and cook until the sugar has dissolved. Swirl the pan gently to move the sugar around, but don’t stir. Add the golden syrup and bubble the mixture (be careful it doesn’t boil over) until it reaches 'hard crack' stage or 150C on a sugar thermometer. If you don't have a thermometer, test the toffee by dropping a small amount into cold water. It should harden instantly and, when removed, be brittle. If it's soft, continue to boil. When it's ready, drip in some food colouring and swirl to combine. Add the glitter, if using, and turn off the heat.