Preheat the oven to 220ºC, fan 200ºC, gas 7. Put the potatoes in a medium pan, cover with cold water and bring to the boil. Cook for 8-10 minutes until tender. Drain and set aside.
Meanwhile, in a large pan, gently heat the lasagne white sauce, then stir in the parsley, watercress, 200g of the peas and the fish pie mix. Season with freshly ground black pepper and cook for 3 minutes.
Stir in the cooked potatoes and divide the mixture between four individual pie dishes.
Cut the filo sheets into quarters. Brush each square with a little of the melted butter and scrunch up on top of each pie filling. Bake for 12-15 minutes until golden.
While the pies are baking, put the sugar snaps and remaining peas into a microwave-proof bowl. Add 2 tbsp of water, cover and cook on high for 2 minutes. Drain and serve with the fish pies.