Sift the flour, mixed spice and salt into a large bowl. Add the butter and, using your fingertips, rub in until the mixture resembles fine breadcrumbs.
Stir in the currants, lemon zest and 80g sugar. Add the egg and mix to a soft but firm consistency. Add a splash of milk to loosen the mixture, if necessary.
Roll out on a floured board to the thickness of a £1 coin. Stamp out rounds using a 5cm fluted cutter.
Heat a griddle or heavy-based frying pan and lightly grease with the oil. Cook the cakes, in batches, for about 3 mins on each side, turning once. They should be dark golden and still a little soft in the centre. Dust with the remaining 20g sugar while still hot and serve straight away.