Put the potatoes in a pan, cover with water, add 1 tsp salt and bring to the boil. Cook for 12-15 mins or until tender enough to smash. Leave to cool.
Heat 1 tbsp of the oil in a frying pan and add the fennel seeds, fennel, chilli and garlic followed by the spring onions. Gently cook for 3-5 mins until fragrant but not coloured. During the last minute, add the samphire.
In a bowl, mash the potatoes until well broken down, then combine with the crabmeat, herbs, spring onion mix, cayenne, lemon zest and mayo. Season and mix until well combined, then chill for 30 mins.
Heat oven to 200C/180C fan/gas 6. Oil your hands and shape the mixture into eight palm-sized patties and dip in plenty of flour. Be careful, as they are quite fragile and sticky – a good way to do this is on a tray lined with baking parchment. If you have time, put in the fridge to set for 30 mins before cooking. Heat the remaining oil in a non-stick frying pan and fry for 2-3 mins on each side until crisp and golden. Transfer to an oiled baking tray and finish in the oven for 5 mins. Serve with mayo, some watercress tossed with the remaining herbs, and the lemon juice with the lemon wedges on the side.