Put the potatoes in a pan of salted cold water. Bring to the boil, then turn the heat down and simmer for 6-8 minutes or until tender. Drain well, leave to steam dry in the pan, then mash coarsely using a potato masher or the back of a fork.
Put the sardines in a bowl, mash lightly with a fork, then add the spring onions, egg, dijon mustard, dill, lemon zest, cayenne pepper and 50g plain flour. Mix until combined, add the mashed potato, season generously with salt and black pepper, then mix again.
With lightly floured hands, shape the mixture into 8 even patties. Put each one on a plate as you work, then loosely cover and chill for 10 minutes.
Heat 1 tbsp of the vegetable oil in a large non-stick frying pan then fry half the fishcakes for 3-4 minutes on each side until golden and piping hot. Put on a plate and keep warm. Repeat with the remaining oil and fishcakes. Serve with salad and lemon wedges for squeezing over.
