Cook the potatoes in boiling, salted water until tender. Drain well then cut into big chunks.
Combine 4 tbsp of the oil with the lemon juice and tarragon and season with salt and pepper. Put the potatoes in a serving dish, pour over half the dressing, mix well and leave to cool slightly.
Meanwhile, heat the remaining tbsp of oil in a frying pan over medium heat and fry the sausages for about 15-20 minutes until cooked. Cut into thick slices on the diagonal. Cool slightly then add to the potatoes with the tomatoes and salad leaves.
Drizzle over the remaining dressing and top with the goat's cheese.