Cook the potatoes in a saucepan of salted water for around 20 minutes, until soft. Drain well and place back in the hot pan. Leave for 5 minutes to dry out, then mash well.
While the potatoes are cooking, cook the haddock. Place the fish in a shallow pan (a small frying pan is ideal). Cover with milk or water and place over a medium to high heat. Bring to the boil, then take off the heat and cover. Allow to sit while you cook the potatoes. The heat from the milk will gently poach the fish.
Spray a small frying pan with low-calorie cooking spray and set on a low heat. Add the spring onions and fry gently for 2-4 minutes until they have softened.
Remove the haddock from the pan and add to the mashed potatoes along with the spring onions, mix well. Season to taste with salt and pepper (don't forget the smoked haddock may be a bit salty so you probably won't need to add too much salt).
Divide the mixture into 12 equal portions. Roll each one into a ball, then flatten it to form a fishcake shape. Put the shaped fishcakes on a plate and put them in the fridge for 20 minutes or so, until the fishcakes firm up a little.
Preheat the oven to 200°C and spray a baking tray with low-calorie cooking spray.
Dip each fishcake into the beaten egg, then lightly coat it in panko breadcrumbs and place onto the baking tray. Once all fishcakes are coated, cook for 20-25 minutes until golden brown.
Add the yoghurt, chive and mustard powder to a small bowl and mix to combine. Season to taste with salt and pepper and serve alongside the fishcakes.