Bring a big pan of water to the boil, add the potato and boil for 8-12 minutes until tender, then drain well.
Meanwhile, melt 25g of the butter in another saucepan. Stir in the flour until it makes a paste, then very gradually whisk or stir in splashes of milk until you have a smooth sauce. Add the stock, then bring to the boil, stirring constantly, until thickened.
Take the sauce off the heat and stir in the lemon juice and zest, parsley and some seasoning. Preheat the oven to 180°C (160°C fan oven) Gas Mark 4.
Randomly scatter the potato, capers, chopped gherkins, peas and fish chunks into a big, deep baking dish. Pour over the sauce.
Melt the remaining 25g of the butter, then brush over the filo pastry sheets. Scrunch up roughly and use to top the pie – you can tear the pastry to fill any gaps.
Bake for 25-30 minutes until the pastry is golden and crisp, the fish is cooked through and the sauce bubbling. Serve straightaway, with the remaining lemon cut into wedges.