Preheat the oven to 180 degrees C and lightly grease a 1.5 litre ovenproof dish with melted butter.
Put the potatoes and a pinch of salt in a saucepan, cover with boiling water and cook for 15 minutes, or until tender.
Meanwhile, melt 40g of the butter in a saucepan. Add the flour and cook, stirring, for 1-2 minutes to make a roux. Whisk in 450ml of the milk, the bay leaf and parsley stalks. Cook over a moderate heat, whisking all the time until the sauce bubbles and thickens. Turn down the heat as low as possible and cook for 2–3 minutes, stirring frequently.
Turn off the heat and remove the bay leaf and parsley stalks. Stir in the fish, prawns, chopped parsley, chives and lemon juice. Season, then spoon into the ovenproof dish.
When the potatoes are tender, drain them well and mash with the remaining butter. Stir in the chopped watercress and 1 1/2 tablespoons milk, then beat with a wooden spoon for a minute or two until fluffy. Season with salt and pepper.
Spoon the mashed potato over the fish mixture, then spread it evenly to the edge of the dish. Fork up the surface. Bake for 25 minutes, or until browned.