The most humane way to kill the crab is to put it to sleep in the freezer for about one hour before cooking. Alternatively you can pierce its underside with a sharp knife or plunge the crab into boiling water.
Put a large saucepan of water on to boil and add plenty of salt. Plunge the crab into the boiling water, place a lid on the pan and boil for 12 minutes. Remove the crab from the water with a large slotted spoon or tongs and leave to cool on a tray.
Peel the potatoes and rinse them very well in a colander until the water runs clear. Cut the potatoes into even-sized large pieces and place in a pan of water. Lightly salt the water and bring to the boil over a high heat. Lower the heat to a simmer and cook the potatoes quite gently for 10-15 minutes or until tender. Drain well and mash. Cover and set aside.
Remove the meat from the crab. Reserve the brown meat for another dish (this can be used to make a pâté by mixing with some Tabasco and Worcestershire sauces and a pinch of ground mace; then cover with clarified butter). Once you have removed all the white meat, pick through it to remove and discard any bits of shell and cartilage. Put the crab meat into a large bowl; cover with cling film and keep cool until using.
Combine the lemon zest, parsley leaves and garlic in a heap on a board and chop together finely. Add to the white crab meat along with the egg yolk, Tabasco, salt and freshly ground white pepper, and beat well until thoroughly mixed. Add the mashed potato to the mixture and beat again, then taste to check the seasoning. Let the mixture sit in the fridge for an hour or so to settle.
Shape the crab mixture into 18 little cakes about 2.5cm/1in in diameter. Lay them on a tray and keep cool until ready to cook.
To make the mayonnaise, beat the egg yolks with the mustard and lemon juice in a bowl, then slowly whisk in the oil. Season with salt and freshly ground white pepper. Stir in the remaining mustard to spike up the taste. Transfer the mayonnaise to a serving bowl.
Lightly flour both sides of the crabcakes. Heat a heavy-based frying pan, then add a little oil and heat until gently bubbling. Gently sit the cakes in the hot oil and fry until pale golden-brown on the base. Turn over and continue cooking for 2-3 minutes or until thoroughly heated, lowering the heat if they start to get too brown.
Place three crabcakes on each plate and put a small bunch of watercress and a lemon quarter alongside. Serve immediately, with the mayonnaise.