Preheat the oven to 180C/350F/gas mark 4
Use a 20cm/8 inch round cake tin.
Grease and line the cake tin. Sift the white flour and wholemeal flour into a large mixing bowl, adding any residue of bran left in the sieve. In another bowl, whisk together the oil, soya milk, orange juice and sugar. Make sure that the sugar has dissolved, then fold in the flour, sultanas and orange rind. Mix well, then spoon into the tin and level the surface.
Bake for about 1 hour, or until the cake feels firm to the touch and is beginning to shrink from the sides of the tin. Leave to firm up in the tin for 10 minutes, then turn out onto a wire tray, peel off the paper and allow to cool.
Store in an airtight tin for 5-6 days, or freeze for up to 3 months.