Place the new potatoes in a pan of salted water and bring to the boil. Boil for for twenty minutes till cooked then drain and allow to cool a little on a sheet of kitchen paper. Place the potatoes on a baking tray, soften the butter and liberally coat the potatoes. Season with salt and freshly ground black pepper then roast for 30 minutes in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC).
Meanwhile, sauté the spinach in a well oiled frying pan. Transfer to a mixing bowl once wilted and squeeze out as much liquid as possible. Finely chop the walnuts and add them to the spinach along with the ricotta. Season with salt and freshly ground black pepper to taste, then mix well until combined.
Wash each cod fillet and dab dry with kitchen paper. Lay each one face down on a long piece of aluminium foil on a baking sheet. Cover each fillet with the spinach-ricotta mixture then roll tightly from the thickest end to the thinnest until the cod is pin-wheeled. Wrap the foil tightly around each one and place the sheet in a moderately hot oven (Mk 5 - 375ºF - 175ºC) for 15 minutes. (If the potatoes are still cooking, leave the oven at the higher setting and place the fish as low in the oven as possible.)
Remove the fish from the oven and allow to rest for one minute before unwrapping. Serve piping hot with the potatoes.