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Heat the oven to 200C/180C fan/ gas 6. Prick the potatoes all over with a fork, rub with the oil, put on a baking sheet and bake for 1 hr 30 mins, or until a knife can be easily inserted. Leave to cool a little.
Put all the crabmeat, the lemon zest, cayenne and mustard powder into a bowl with a large pinch of salt. Halve the potatoes and scoop the flesh into the bowl. Gently fold everything together, then form into 12 small patties using your hands.
Tip the flour into a shallow bowl, pour the egg into a second, and the breadcrumbs in a third. Dip each patty into the flour, then the egg, and finally, roll in the breadcrumbs. Put the fishcakes on a plate and chill for 10 mins.
For the tartare sauce, combine all the ingredients in a bowl. Set aside.
Heat the oven to 200C/180C fan/ gas 6. Heat a good glug of oil in a non-stick frying pan over a medium heat, and fry half the crab cakes for 3 mins on each side, or until golden and crisp. Transfer to a baking sheet and bake for 8-10 mins, or until cooked through.
Serve the crab cakes on a platter with a bowl of the tartare sauce on the side for dipping.