Preheat oven to 190°C (170°C for fan assisted ovens)/ 375°F/Gas M5.
Cut Leek in to four lengthways, then each quarter into four, lay on baking sheet and drizzle lightly with butter and bake for 8-10 minutes until lightly cooked. Allow to cool.
Cut pastry into three equal sections across the width to give 3 long strips. Cut these 3 sections into 5 squares.
Wrap a piece of ham around each leek segment and brush each pastry square lightly with butter. Place the wrapped leek diagonally across one square and roll the pastry into cigar shape. Brush again, sprinkle with sesame seeds and bake for 8-10 minutes.