Preheat the oven for 160°C (325°F).
Soften butter and muscovado sugar in a mixing bowl. Gradually mix in the eggs and mix well. Sift in the flour and salt and fold in with a large spoon. Add in the cider and apples when flour is only partially mixed in. Grease the springform tin and line the base with baking paper.
Pour the mixture into the form and sprinkle with demerara sugar. Bake for 45-60 minutes until golden brown.