Scrub the potatoes, then cook in a pan of boiling salted water for about 15 minutes, or until lovely and tender when pricked with a fork. Drain in a colander and leave to steam dry.
Transfer the potatoes to a large bowl and gently crush a few with a fork to add texture. Add the butter and season well with sea salt and black pepper.
Flake in half of the smoked trout, add the cider vinegar or grate in the lemon zest and squeeze in half the juice. Toss through.
Serve at room temperature, with the remaining trout flaked over and chives snipped on top. Delicious served on its own or with some soft round lettuce.