Grease two 7" square pans and line them with baking parchment.
Cream the butter and sugar until light and fluffy.
Sift the flour with the cream of tartar into a separate bowl and add the raisins.
In a third bowl, combine the milk, sherry, spices, syrup, treacle and eggs.
Fold the flour mixture alternately with the milk and spice mixture into the butter and sugar. Beat until thoroughly combined. Beat in orange rind and the dissolved soda.
Pour into the prepared pans and bake in a preheated 325F (160C) oven for thirty minutes, reduce oven to 300F (150C) and continue baking for 30 - 40 minutes more. Allow to cool in the pans, then turn out onto a rack and allow to finish cooling.
To serve, cut into small squares and serve with butter.