Mash one of the bananas into small bowl, then mix in the sugar and milk.
In a large mixing bowl, sift in the flour and baking powder. Mix in the oats and poppy seeds.
Pour in the banana mix, folding in well. Stir in the yogurt.
Carefully pour the mixture into a lined traybake and level out. Chop the other banana into 24 thin slices and arrange on top, so that there'll be 2 slices per bar.