Put the currants and sultanas in a bowl and pour over the hot tea. Stir, cover and leave to soak overnight for the liquid to absorb.
Preheat the oven to 150C/130C Fan/Gas 3½. Grease a 900g/2lb loaf tin and line with non-stick baking paper.
Add the flour, sugar and egg to the bowl of soaked fruit (if there is a little excess liquid this is fine) and mix thoroughly.
Spoon into the prepared tin and level the top. Bake for 1¾ hours, or until the cake is risen and just firm to the touch, but check it after 1¼ hours to see how it is doing. Leave to cool in the tin for about 10 minutes.
Turn out the cake and remove the paper. To serve, cut into fairly thick slices and spread with butter to serve.