Unroll the pastry with the short end facing you. Divide the anchovies into 2 piles. Brush half the pastry with the butter.
About 5cm from the bottom lay the anchovies end to end across the pastry horizontally so you have a line of them left to right. Add any spare into the line. Repeat 10cm away from you. Fold the plain pastry half over the anchovies and press