Thickly slice the sweet and baking potatoes, then cut into thick wedges. Mix 3 tablespoons of the oil with 1 teaspoon paprika, 1 teaspoon oregano, ½ teaspoon mustard, ½ teaspoon chilli seeds and some salt in a large plastic bag or bowl. Add the potatoes and toss in the oil mixture.
Mix the remaining paprika, oregano, mustard, chilli seeds and seasoning with the flour on a large plate. Beat the eggs and measured water in a shallow dish and put the breadcrumbs on a second large plate.
Coat the chicken pieces in the flour mixture, then the beaten egg, then the breadcrumbs, until completely covered.
Heat a large roasting tin in the oven, 200°C (400°F), Gas Mark 6, for 5 minutes. Meanwhile, heat the remaining oil in a large frying pan, add the chicken and fry until pale golden. Transfer the chicken to the hot roasting tin, add the potatoes and roast for 30–35 minutes until the chicken is cooked through and the potatoes crisp and golden. Transfer to serving plates and serve with mayonnaise and salad.