Put the liquid, peel, sugar and lemon juice in a very large, wide saucepan or preserving pan and gently heat while stirring to dissolve the sugar. Skim off any scum, then boil until the marmalade reaches 105/106C on a sugar thermometer. At this point it should form a puddle on a chilled saucer, that wrinkles when you push your finger through it. If it doesn’t, keep simmering, and skimming off the scum until it does. Turn off the heat, stir in the whisky, if using, and leave it to rest for 15 minutes.