Quarter the apples and remove the core from each piece. I don’t peel the apples as I love the pink colour of the skin but peel them if you prefer. Cut each quarter into slices, about 3mm thick at the thickest part. Put them into a saucepan with the cordial and water, cover and simmer the apples very gently until they are just beginning to get tender on the outside. Strain off the liquid and measure how much you have. If it is more than about 75ml then put it into a small saucepan and boil to reduce it to 75 ml. Stir in the zest, pour the liquid over the apples and leave to cool. Preheat the oven to 190 C and put a metal baking sheet into it.