Peel the potatoes and cut them into chunks. Place in a large pan and fill with enough cold water to cover the potatoes.
Put the pan on high heat, bring it to a boil and turn the heat down to the simmering point. Gently simmer for 20-25 minutes or until soft in the middle.
Start your fish pie filling while the potatoes are cooking.
Place a pan on medium heat and melt the butter. Add the leeks and saute until soft.
Sprinkle on the flour and stir to combine. Cook for a minute and whisk in a little of the warm milk at a time, stirring continuously until you have a smooth and velvety mixture. Cook for a few minutes until the mixture is thickened.
Add the fish mixture, dill, tarragon, peas, freshly ground black pepper, and salt, give it a good stir, and remove from the heat.
Spoon the mixture into a pie dish size of 25-27 cm (10 ") diameter (or any similar-sized rectangular or oval oven dish).
Preheat the fan oven to 180° C (360° F) and start preparing the mashed potatoes.
When potatoes are soft in the middle, drain well and return them back to the pan.
Put the pan back on low heat and let all the water evaporate for 30 seconds then remove from the heat.
Add the double cream, butter, and salt. Mash using a masher or a ricer until smooth.
Stir in the grated cheddar cheese and start assembling the fish pie.
Spoon the mashed potato on top of the fish mixture and spread it out evenly using a fork or pallet knife. Give it a lined pattern to help the mash crisp up.
Place the dish in the oven and cook for 30-35 minutes until the top is golden and the filling is bubbling.