Cook the potatoes in a pan of boiling water for 15 minutes, or until tender. Drain, then return the potatoes to the pan and heat for one minute to drive off any excess moisture. Mash the potatoes, or pass through a potato ricer into a bowl. Set aside to cool slightly.
Stir in the parsley, spring onions, lemon zest and juice until well combined. Season with salt and pepper. Fold in the haddock.
Divide the mixture into 8 equal portions. Place a soft-boiled quail's egg in the middle of 4 portions and cover them with the remaining portions. Shape into 4 balls.
Half fill a large saucepan with the oil and place over a medium heat. If you're using a deep-fat fryer or have a thermometer, heat the oil to 180C. If not, to check the oil is at the right temperature, drop a 2-3cm/¾-1¼in cube of bread into the hot oil. It should be golden-brown and crisp within 1 minute (CAUTION: hot oil can be dangerous. Do not leave unattended).
Put the flour, beaten egg and breadcrumbs into 3 separate bowls. Dredge the fish cakes in flour, dip in beaten egg and finally coat in the breadcrumbs. Deep fry them (in batches if necessary) for 2 minutes, or until golden-brown. Drain on kitchen paper.
For the mayonnaise, place the egg yolks, vinegar and mustard in a food processor and blend. With the motor running slowly add the oil in a thin stream until emulsified. Remove from the blender and stir in the curry powder, turmeric and lemon juice. Season with salt and pepper.
For the garnish, place the watercress in a large bowl. Place the vinegar, mustard and oil in a small bowl and whisk to combine. Just before serving add the dressing and mix.
Place the mayonnaise in the centre of each plate and top with the fish cakes and garnish with dressed watercress.