Roughly chop the fish, then peel and grate the potato. Combine with the flour in a bowl, then season with sea salt and black pepper. Bring together with your hands and shape into 12 flat patties, then leave to chill in the fridge. Heat a glug of oil in a heavy-based pan over a medium-high heat. In batches of 4, cook the fish cakes for 2 to 3 minutes on each side, until golden and crisp, then remove to kitchen paper to drain. Pick, chop and stir the herbs through the mayo, then serve with the fish cakes, and some watercress and lemon wedges, if you like.