Boil potatoes until tender, mash and add 1 tablespoon butter and ¼cup/60ml of milk.
Preheat oven to 375F/190C.
Poach haddock in the wine and 1 ¼c/300ml milk over a medium-low heat for around 10 - 15 minutes until cooked through (mine didn't even get to simmer). Don't let it get above a simmer. Add the shrimp towards end if not already cooked. Remove fish from milk mixture.
Melt 1 tablespoon butter, add flour and cook until nutty, gradually add milk-wine mix from poaching to make sauce, add in dill and wilt in spinach.
Put flaked fish and shrimp in baking dish, pour over sauce, top with mashed potatoes. Bake approximately 30 minutes until starting to brown.