Preheat the oven to 160C/325F/gas mark 3
Use a 15cm/6 inch square tin.
Lightly grease the cake tin. Put the raisins, currants and walnuts into a small mixing bowl and mix thoroughly. Sift the flour and baking powder into another bowl and rub in the butter until the mixture resembles coarse breadcrumbs. Add the fruit and nuts, treacle, milk and sugar. Using a spatula, fold so that there are no dry pockets of flour. Lastly, very lightly fold in the lemon juice.
Pour this mixture into the tin and level it off. Bake for about 1 1/4 to 1 1/2 hours, or until the cake is shrinking from the sides of the tin. Allow to firm up in the tin for 10 minutes, then turn out, peel off the lining paper and cool on a wire tray.
