Step 1 - Preheat the oven 200C/Gas 6
Step 2 - Remove any bones from the fish.  Poach the fish for 5-10 minutes in the milk with the bay leaf, until the fish is cooked
Step 3 - Once the fish is cooked lift out the fish keeping the cooking liquor.  Allow to cool and remove all the skin and bones, flaking the fish as you go
Step 4 - Peel and cut the potatoes into peices and cook.  Mash until smooth with a little butter
Step 5 - Strain the milk from poaching the fish and make up to a pint with more milk. 
Step 6 - To make the sauce, melt the butter in a small saucepan and combine with the flour.  Once the butter and flour are blended together and cooked out, add the milk gradually until the sauce is thickened.  Mix in the cheese and chopped parsley.
Step 7 - Place the flaked fish in the bottom of your pie dish.  Scatter the peas on top of the fish and then pour in the sauce.
Step 8 - Spread the mash potato over the top then sprinkle over the crushed Cheese and Onion biscuits.
Step 9 - Cook in a preheated oven for 20 minutes until golden brown on top