Cut any large potatoes in half. Bring a pan of salted water to the boil and add the potatoes. Bring back to the boil and simmer for 15 minutes until nicely cooked. Meanwhile, make your dressing by mixing the yoghurt with the lemon zest and juice, a good pinch of sea salt and black pepper and a lug of extra virgin olive oil. Drain the potatoes well, tip into a mixing bowl and season immediately with salt and pepper. Drizzle with a little olive oil and, after 5 minutes, stir in most of the dressing. Leave the potatoes to cool. Peel and halve the cucumber lengthways, remove the seeds and cut into ½cm slices. Roughly chop the fennel. Add the cucumber and fennel to the potatoes, pick in the basil leaves and mix well. Divide the salad evenly between four plates and flake over the salmon. Drizzle with a little olive oil.