For the marinade, place all of the marinade ingredients into a small bowl and whisk well to combine.
For the duck, place the duck legs into a bowl and pour over the marinade. Cover with cling film and place into the fridge to marinate for at least 24 hours.
For the pickled radish salad, place all of the salad ingredients into a bowl and mix well to combine. Cover with cling film and set aside in the fridge for one hour.
Preheat the oven to 180C/350F/Gas 4.
For the duck, remove the duck legs from the marinade and place, skin-side up, into a roasting tin. Place into the oven to roast for ten minutes, or until the skin is golden-brown and crisp.
Increase the oven temperature to 220C/425F/Gas 7.
Roast the duck legs for five more minutes, or until the duck is cooked to your liking. Remove from the oven and allow to rest for 2-3 minutes.
When just cool enough to handle, cut the duck into bite-sized slices and lightly dust in the potato flour, shaking off any excess.
Half-fill a wok or deep heavy-based pan with the groundnut oil and heat to 180C/350F, or until a cube of bread dropped in sizzles and turns golden-brown in 15 seconds. (CAUTION: hot oil can be dangerous. Do not leave unattended.) Carefully lower the duck slices into the oil and fry, in small batches, until crisp and golden-brown. Remove from the pan with a slotted spoon and drain on kitchen paper.
To serve, spoon some pickled radish salad onto each of two serving plates and arrange the crispy duck slices on top. Sprinkle over a pinch of salt and some dried chilli flakes over the duck pieces and serve immediately.