Oven at 180c Fan
Place the fish in an oven proof dish with the milk. The milk should reach half way up the fillets, don't add all of it if not required.
Put in the oven for 20 minutes
Warm peas (esp if frozen!) in hot water, do not boil
Boil and mash potatoes while fish is in the oven
Remove fish from oven after their time, increase oven to 220c
Add butter and some milk from the fish pan to the mash to make it smooth and creamy, then gently mix in the peas
Drain the fish
Place fish in the final pie dish
Add the prawns
Sprinkle with the herbs
Add mash to the top of the dish
Cover with the cheese
Put in the oven for 20 minutes
Serve!