Heat the oven to 150C (130C fan). Grease a 900g loaf tin and line the base with baking paper.
Brew the tea bag in 250ml of boiling water for 5 minutes, then stir in the malt extract, black treacle and brown sugar until dissolved. Leave to cool for 5 minutes.
Put the flour, salt and sultanas in a mixing bowl and make a well in the middle. Pour in the warm tea mixture and beat with a spoon until you have a thick, sticky batter with no dry pockets of flour.
Spoon into the tin and level the top. Bake for 55 to 60 minutes until firm to the touch and a skewer comes out clean.
Cool in the tin for 10 minutes, then turn out onto a rack. Wrap the cold loaf in baking paper and foil and keep it a day before slicing, if you can wait: it becomes stickier and better overnight.