In a large mixing bowl mash together the yeast and 3 tablespoons caster sugar with a spoon. Once well combined, add the milk and 3 tablespoons plain flour; whisk well. Cover with cling film and let stand for 20 minutes till risen.
Sift 500g flour into the yeast mixture. Add 4 tablespoons sugar, melted butter, water, egg and salt. Using your hands, combine and knead well. Cover and let rise for 30 minutes, till about doubled.
Combine the streusel ingredients with a fork till crumbly. Set aside.
Preheat the oven to 180 C / Gas 4. Grease a large baking tin, approximately 25x40cm.
Dust a work surface with flour. Remove dough from bowl and roll out to approximately the same size as the baking tin. Transfer to the tin, then cover with the plums. Sprinkle over the streusel topping, then let rise for 20 minutes.
Bake in the preheated oven for 1 hour. Remove and let cool slightly.
Prepare the icing: Add a bit of the warm water to the icing sugar and mix well. Add more water as needed to achieve a drizzling consistency. Squeeze in some lemon juice to taste.