Put the flour, sugar, butter and ½teaspoon salt in a food processor. Process in short bursts until the butter is incorporated. With the machine running, gradually pour in the milk and process until the dough comes together — you may need to add an extra 1tablespoon milk. Turn out onto a lightly floured surface and knead briefly until smooth. Cover with plastic wrap and refrigerate for 1 hour.
Preheat the oven to 190°C (375°F/Gas 5). Lightly grease a large baking tray. Cut the dough into four portions. Working with one portion at a time, roll until very thin, into a rough square, measuring about 20cm (8inches) along the sides. Place a dough square on the greased tray, brush the top lightly with water and sprinkle with the sesame and poppy seeds. Roll a rolling pin lightly over the surface of the dough to press in the seeds.
Bake for 6–8 minutes, or until golden brown and dry. Repeat the process with the remaining dough. Transfer to a wire rack until cool and crisp. Break into large pieces.