Preheat the oven to 190 degrees C, Gas mark 5. Place the potatoes in a large pan of cold water, bring to the boil then add the sweet potatoes. Simmer for 15 – 20 minutes until both are tender.
Place the bacon in a wide heavy based pan and cook gently until starting to release its fat, increase the heat and fry for 5 minutes until crisp. Stir in the salad onions and sweetcorn, cook for a further minute then transfer to a pie dish about 25cm square and 5 cm deep.
Pour the milk into the bacon pan and add the cod and prawns. Bring to the boil and poach gently for 5 – 6 minutes until the fish starts to turn opaque. Transfer the cod and prawns to a plate and strain the milk into a jug.
In a medium heavy based pan, melt 50g of the butter and stir in the flour using a wooden spoon until it is a thick paste, remove from heat and add a little of the warm milk stiring until smooth, repeat this process until all the milk is used. Return the pan to the heat and stir constantly until it comes to the boil. Simmer for 2 minutes then add the cod and prawns, discarding any liquis left on the plate. Season, pour into the pie dish and stir through.
Drain potatoes then mash with the remainder of the butter and season. Pile the mash over the filling, spreading to the edges with a fork.
Place in the oven and cook for 30 minutes or until piping hot and the top is browning, then serve with peas or salad.
*NOTE* You can make the pie ahead, cover and chill. When ready to cook, place in a preheated oven at 180 degrees c, gas mark 4 for about 45 minutes.