Cut the lamb from the bone into 4 cm (1 1/2 in) cubes, discarding the gristle and a little of the fat. Season generously with salt and freshly ground black pepper.

In a flameproof casserole, heat the olive oil and butter. Saute the lamb in 2 - 3 batches, 1 - 1 1/2 minutes each side, or until lightly browned. Remove with a slotted spoon to a plate and keep it warm. Repeat with the remaining meat.

Add the finely chopped onion, celery, pepper and garlic to the casserole and cook over a moderate heat for 10-12 minutes or until softened, stirring occasionally.

Add the curry paste, ginger, turmeric, paprika, cayenne and flour to the softened vegetables and blend with a wooden spoon. Add the chicken stock and sultanas; bring to the boil, stirring constantly. Simmer gently for 10 minutes.

Return the lamb to the casserole, cover with a lid and simmer gently for 1 hour, or until tender, stirring occasionally.

Stir the yoghurt and lemon juice into the curry. Heat through and serve with rice.