Place all the ingredients in a large bowl and rub together until you get crumb like texture. Set aside.
Crust:
In a large bowl, beat butter until soft and creamy. Add sugar and continue beating until light and fluffy. Add egg and beat well. Add sour cream and again beat until well combined. Sift in flour, baking soda and mixed spices. Mix until combined.
Pour this batter into a baking tray/sheet pan (13x9inch/33x23cm) inch pan lined with baking parchment paper. Cover the whole surface with redcurrants (stems removed) and scatter the crumb topping over them.
Bake in a preheated oven at 356°F/180°C for 60 minutes or until baked through.