For the roast shoulder of lamb, preheat the oven to 170C/325F/Gas 3.
Rub the shoulder of lamb all over with the olive oil, then season with salt and freshly ground black pepper and set aside.
Meanwhile to make the Madras curry powder, place the coriander, fenugreek, mustard, cumin, fennel seeds, black peppercorns, cinnamon and cloves in a spice grinder and grind to a powder. Stir in the turmeric.
Put the chilli, garlic and ginger in a small food processor and blend until a paste if formed. If it does not come together add a small amount of water.
Heat the vegetable oil in a casserole pot with a lid. Add the lamb and fry on all sides until browned. Remove the lamb from the casserole and set aside.
Add the onion to the hot casserole pot and fry until soft and starting to colour. Add 2½ tbsp of the Madras curry powder, then the garlic, chilli and ginger paste and the curry leaves, cardamom and bay leaves and stir.
Add the lamb back into the pot and cover with the chopped tomatoes, stock and tamarind and season with salt and pepper. Place a lid on and roast the lamb in the oven for 3-4 hours, basting every hour or so with the juices in the roasting tray. Cook until the meat is very tender and falling off of the bone.
Once cooked remove the lamb from the sauce and simmer the remaining liquid until it reduces in volume to make a thick sauce.
Using a fork, shred the lamb and add it back to the sauce.
For the Bombay potatoes, place the potatoes in cold water and bring to the boil. Cook for 5-8 minutes and drain.
Heat a large pan and add the oil. Once hot, add the mustard seeds. When they start popping in the pan add the chilli powder and turmeric and add the potatoes. Take care of your eyes and face while the mustard seeds are popping.
Cook for 5-8 minutes, or until the potatoes absorb the spices.
To serve, place the lamb Madras on a serving plate with the potatoes alongside. Garnish with chopped coriander.