Pre-heat your oven to 160c fan / 180c / 350F / gas 4
Place the sultanas in a heatproof bowl and add enough boiling water to just cover them. Leave to stand for 20 minutes then drain and blot dry with a clean tea towel. Return the sultanas to a dry bowl.
Grease your cake tin and line the base with baking parchment.
Take 1 tablespoon of you measured flour and sprinkle over the sultanas. Thoroughly coat the sultanas with the flour, this will help stop them from sinking in the cake mixture during baking.
Sieve together the remaining flour and baking powder into a bowl and leave to one side.
In a large mixing bowl, cream together the butter and the caster sugar until they are light and fluffy. You can do this with the wooden spoon or save time and use an electric hand mixer or a stand mixer.
Slowly add the eggs and beat well between each addition. As more egg is added the mixture may look like it will curdle so add a spoonful of your measured flour
Add the sieved flour and baking powder to the butter and sugar mixture and gently fold them together using a spatula or metal tablespoon. Do this gently to keep as much air in the creamed mixture as possible.
The mixture should be stiff but still drop off the spoon in about 5 seconds. If it's too stiff, add a little milk.
Scatter over the sultanas and gently stir them into the mixture evenly.
Spoon the mixture into your prepared cake tin and level the top.
Sprinkle the demerara sugar evenly over the top of the cake and bake in the centre of the oven for about an hour until it is well risen and golden brown
Check the cake by inserting a skewer in the centre, there should be just a few crumbs sticking to the skewer. If there is still raw mixture, continue baking for another 15 minutes and check again.
When the cake is cooked, leave in the tin to cool for 20 minutes then remove from the tin, remove the baking parchment and place on a wire rack to finish cooling.