To make the sauce, first reduce the stock to about 200ml. Then, simmer it in a heavy based pan with the double cream until it's the consistency of cold pouring cream. Test the seasoning and stir in the herbs. Season with salt and pepper. Sweat the leeks in 25g of butter until completely tender.
Mix the fish, peas, leeks and cream sauce, and put in an appropriate ovenproof dish. Chill until it's set (it makes it easier to put the mash on if the fish mix is cold).
Preheat the oven to 200C/400F/gas mark 6.
Warm the milk and remaining butter and incorporate into the mashed potato and mix until smooth. If you have a piping bag use it to pipe the mash on top of the fish mixture. If you don't have a piping bag use a spatula or palette knife to apply the mash and ruff up with a fork.
Bake the fish pie for 35 mins. Then check the temperature by holding a small knife to the middle of the pie for 10 seconds – if it's nice and hot it's ready, if not return to the oven for enough time to make the pie hot to the very centre. Leave to stand for a few minutes before serving.