Heat the oven to 200°C/180°C fan. Put the potatoes in a saucepan and cover with water, bring to the boil and cook for 15-20 minutes until very tender. Drain, then return to the pan over a medium heat for 30 seconds to dry. Mash with half the butter, 250ml milk and two thirds of the cheese.
In a saucepan melt the remaining 100g butter and add the flour to the pan, cook for a minute or two more then gradually stir in the remaining 500ml milk and cream to form a thick, shiny, sauce. Stir in the mustard and season to taste.
Turn off the heat, then stir in the spinach and allow to wilt. Mix the prawns, fish, capers, and parsley into the sauce. Pour over the sauce and mix well. Top with the mashed potato, then scatter over the remaining cheese.