Combine the lamb and buttermilk in a deep baking dish, cover tightly and marinate overnight.
Pre heat oven to 160°C. (140°C if fan forced).
Uncover lamb, add water, season well with salt and pepper.
Bake for 3 1/2 hours, basting every hour or so with the cooking liquid.
Remove from oven, cover and rest for 10 minutes.
I served mine with a herb salad and yoghurt.
The cooking liquid will make an amazing gravy, just add some flour and stock if needed.