Fry the sausages in a large heavy-based saucepan set over a moderate heat in half the oil for 8-10 minutes, turning often, until browned and firm to touch. Set aside.
Sauté the vegetables in the remaining oil for 5 minutes, until lightly softened, then add the curry powder, stock, sultanas and apple. Bring to a simmer. If using a slow cooker, transfer to cooking insert.
Add the sausages and cook gently for 1 hour in the saucepan, or 4 hours in the slow cooker on low heat. Fold in the peas and cream, simmer briefly, then serve.