Scrub the potatoes, then cook in a pan of boiling salted water for 15 minutes, or until tender, then drain and steam dry.
Fill a large pan (big enough to hold all the salmon fillets in one layer) with boiling salted water and place on a medium heat.
Gently lower in the salmon fillets and poach very gently for 10 minutes, then remove to a plate with a fish slice and leave to cool.
Pick and finely chop the dill, then pick and tear the basil leaves. Deseed and very finely slice the chilli. Finely grate the lemon zest.
When cool enough to handle, slice the cooked potatoes in half lengthways and toss with half the lemon juice, 1 tablespoon of oil, the chopped dill and a pinch of sea salt and black pepper.
Arrange on a serving plate and flake the cooked salmon over the top.
Season the yoghurt to taste, then mix in the lemon zest and remaining lemon juice.
Spoon the yoghurt over the salmon, scatter over the basil and chilli, then snip over the cress.