Cook the potatoes in lightly salted boiling water for about 20 minutes. Mash until smooth with the milk and 50 g (2 oz) of the butter. Season well.
Meanwhile, melt the remaining butter in a large frying pan, add the haddock fillets, skin-side down, and cook for about 3 minutes until golden and crispy. Carefully turn over the fillets and cook for a further 1–2 minutes.
Remove the fillets and transfer to serving plates with the mashed potato.
Return the pan to the hob. Increase the heat until the butter turns nut-brown in colour, then add the capers and lemon juice. Bubble for a minute and then spoon over the fish and potatoes. Serve immediately.