Boil the potatoes for 12-15 mins until just tender. Meanwhile, flake the mackerel fillets into large pieces and cut the beetroot into bite-size chunks.
Drain the potatoes and cool slightly.
Mix the salad dressing and horseradish sauce together in a salad bowl and season. Tip in the potatoes they should still be warm.
Add the salad leaves, mackerel, beetroot, celery and walnuts, and toss gently.