Cook the potatoes in lightly salted boiling water for 10 minutes, until tender. Drain, mash and set aside to cool down.
Beat the tuna, Cheddar, spring onions, garlic, thyme and egg into the mashed potato, add the cayenne pepper and season with salt and pepper.
Divide the mixture into 8 and make into thick burgers. Sprinkle the flour over them and fry in a shallow layer of hot oil for 5 minutes on each side, until they are crisp and golden. Serve with lemon wedges and watercress.