First make the pastry. Sift the flour into a large bowl, add the butter and, using your fingertips, rub it into the flour so the mixture resembles breadcrumbs. Measure 150ml/¼ pint cold water into a jug. Stir the ground almonds and sugar into the flour mixture, then add the egg yolk and a little of the water, stirring with a knife. Gradually work in the rest of the water, stirring until you have a soft pastry ball. Wrap it in cling film and chill for 20-30 minutes.
Meanwhile, make the filling. Preheat the oven to fan 180C/conventional 200C/gas 6. Tip the ground almonds, sugar, currants, sultanas, almonds and spices into a large bowl and stir. Melt the butter (use the microwave for speed) and add to the dry ingredients with the beaten eggs and the lemon and orange zest and juice. Mix together really well.
Roll out the pastry on a lightly floured surface (working with half at a time is easiest) to about 5mm thickness. Cut out about 24 rounds using a 10cm round cutter. Use each round to line a muffin tin (use two trays, or bake in batches). Spoon the filling into each pastry-lined tart, so it is just shy of the top of the pastry.
Bake for 20-25 minutes until pale golden, then turn each tart out on to a wire rack and dust with icing sugar. Lovely served warm or cold.