Grease and line a 23 cm (9 inch) round or 20 cm (8 inch) square cake tin, using a double thickness of greased greaseproof paper. Line the outside with several thicknesses of brown paper, standing at least 5 cm (2 inches) above the top of the tin.
Sift the flours into a bowl with the salt, mixed spice, cinnamon and nutmeg. In another large bowl, cream the butter with the sugar until light. Beat in the treacle.
Lightly beat together the eggs, sherry or tea and vanilla extract. Gradually beat half the egg mixture into the creamed mixture. Fold in one-third of the mixed flours. Continue to add the egg and flour mixtures alternately. Mix in all the remaining ingredients except the brandy. Turn into the prepared tin and smooth the top.
Bake in a preheated oven, 140°C (275°F), Gas Mark 1, for about 3–3½ hours until a skewer inserted into the centre of the cake comes out clean. Cover the cake with a double layer of greaseproof paper if it starts to brown too much during cooking.
Leave the cake to cool in the tin before turning out on to a wire rack to cool completely. Prick all over with a fine skewer and spoon brandy over the cake. Store the cake in an airtight tin and leave to mature for about 1 month before using.