Remove the stalks from the apples, then put them in a heatproof bowl and pour over boiling water from the kettle to cover them and leave for 3-4 mins. Remove with a slotted spoon and pat dry. (This removes the protective wax from the skin and makes the toffee stick to the apples better.)
Push the sharpest end of each stick into the stalk-end of each apple, making sure it's firmly wedged in. Put a large piece of baking parchment on a board.
Tip the sugar into a large saucepan, 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 simmer 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, if you like, and swirl to combine. Turn off the heat.
Working quickly, dip each apple into the toffee, tipping the pan to cover it fully. Lift out and allow any excess to drip off back into the pan before placing on the baking parchment. Repeat with the remaining apples. Gently heat the toffee again, if you need to. Leave to set. Best eaten on the same day.