For the Slattern's Spaghetti, and whenever anchovies are mentioned, are we talking fresh or tinned anchovies? It's never stated.
Nigella's Slattern's Spaghetti (from KITCHEN) is Nigella's version of the classic Italian dish pasta alla puttanesca. The sauce is a combination of salty, spicy and umami, which comes from the chilli flakes (red pepper flakes), olives, capers and anchovies.
In this dish, and in all of Nigella's recipes, the anchovies are the type sold in jars or cans and packed in oil, unless the ingredients list specifies otherwise. Before being packed in oil the anchovies are salt cured, which intensifies the flavour and the oil-packed anchovies are usually fillets rather than whole fish. You can keep the drained oil to whisk into vinaigrette dressings or to use to cook onions for a stew or braise. The whiter, milder pickled anchovies, such as boquerones, are better eaten as they are, maybe as an aperetif.