Method

1. Preheat the oven to 200°C/gas mark 6/400ºF. Line a 12-bun muffin tin with muffin papers or (as I have here) silicone inserts.
2. Measure the flour, baking powder, bicarbonate of soda, caster sugar, cinnamon and nutmeg into a large bowl; grate the zest of the clementine/satsuma over, and combine. If you are doing this in advance, leave the zesting till Christmas morning.
3. Squeeze the juice of the clementines/satsumas into a measuring jug, and pour in the milk until it comes up to the 200ml mark / halfway between the ¾ cup and 1 cup marks.
4. Add the oil (or slightly cooled, melted butter) and egg, and lightly beat until just combined.
5. Pour this liquid mixture into the bowl of dried ingredients and stir until everything is more or less combined, remembering that a well-beaten mixture makes for heavy muffins: in other words a lumpy batter is a good thing here. 
6. Fold in the cranberries, then spoon the batter into the muffin cases and sprinkle the demerara sugar on top.  Bake in the oven for 20 minutes, by which time the air should be thick with the promise of good things and the good things themselves golden brown and ready to be eaten, either plain or broken up and smeared, as you go, with unsalted butter and marmalade.