Preheat the oven to 200C/180C Fan/Gas 6.
Put the potatoes in a saucepan of salted water and bring to the boil. Boil for 15 minutes, or until tender. Drain well and mash. Season with salt and pepper and set aside to cool.
Grease a sheet of foil with the butter and put the cod loin in the centre, season with salt and pepper and wrap in the foil to make a parcel. Put on a baking tray and cook for 20 minutes, or until just cooked through. Set aside to cool. When completely cool, carefully flake the fish into large pieces. Reserve the cooking juices.
Spoon the cold mash into a bowl, mix in the spring onions, mayonnaise and mustard. Add 2 tablespoons of the fish cooking juice, then add the flaked cod and stir in the crabmeat. Add the parsley and hot pepper sauce and stir until combined. Season to taste.
Shape the mixture into 8 fishcakes, roughly 9cm/3½ in diameter, this is easiest with damp hands. Place on a tray and chill in the fridge for at least 15 minutes.
Sprinkle the breadcrumbs on a plate and coat each fishcake. Return to the tray and chill again in the fridge for 30 minutes.
Meanwhile for the salsa, mix all the ingredients together and season with salt and pepper.
Heat a wide, heavy-based frying pan, add the butter and oil and fry the fishcakes over a high heat for 4 minutes on each side, or until golden-brown all over. When frying, it’s best not to overcrowd the pan, cook in two batches, if necessary.
Serve hot with the tomato salsa and a simple dressed salad.