Peel both types of potatoes and cook in a saucepan of lightly salted boiling water for about 15 minutes or until soft when pierced with the tip of a knife. Drain well, return to the pan and roughly mash. Allow to cool.
When the potatoes are cold, add the beaten egg, season with salt and pepper and mix well. Add the crab meat and mix into the potato mash with the paprika and flour.
With floured hands, shape 2 tablespoons of the mixture into a flat cake. Repeat until all the mixture has been used, then chill the crab cakes until required.
When ready to serve, heat the oil in a large frying pan and fry 4 crab cakes at a time for 3–4 minutes, turning occasionally, until they are golden brown on all sides and heated through. Remove from the oil and drain well. Keep warm while cooking the rest of the crab cakes. Serve immediately with garlic mayonnaise and crisp salad leaves.