Make the mayonnaise. Beat the egg yolks in a bowl with the mustard and lemon juice. Add the oil, drop by drop, beating constantly until it is incorporated and you have a thick, smooth emulsion. Season and stir in the herbs, watercress, lemon rind and garlic, adding extra lemon juice to taste. Cover and chill until required.
Wash the squid and pat dry with kitchen paper. Cut the bodies into rings about 2 cm (¾ inch) thick. Mix together the flour, paprika and cayenne and season well. Put the flour in a plastic bag, add the squid rings and tentacles and shake until they are coated.
Heat the oil in a large frying pan or deep-fat fryer to 180°C (350°F) or until a cube of bread browns in 20 seconds. Remove about one-third of the squid from the bag and shake off the excess flour. Carefully drop the squid into the oil and fry for 2–3 minutes until golden and crispy, then remove with a slotted spoon. Drain on kitchen paper and keep them warm while you cook the rest.
Transfer the squid to 4 serving plates, sprinkle with parsley and serve immediately with lemon wedges and Lemon and herb mayonnaise.