Melt the butter in a large lidded saucepan or casserole dish, add the potatoes and leeks. Season with salt and pepper and cook for a few mins, stirring regularly.
Add the tarragon and pour in the white wine and cook for 5mins. Add the stock and bring to the boil.
Add the peas, cover the pan and turn down the heat and simmer until the vegetable are tender, this should take around 20mins.
Add the double cream, place the cod fillets on top and cook for a further 10 - 15mins until the cod is cooked through. Serve immediately.