Did you wrap the roasting tin tightly in foil?
The way I do it is. I rub with the Lamb oil and add garlic, rosemary or whatever I fancy. Then I put the lamb in a hotish oven for 15-20 minutes to brown. You could also brown in a frying pan to seal it if you like. . Then the trick is, to put a layer of foil over the lamb in the roasting dish. Make sure the foil is tightly wrapped around the edges of the dish so that no steam can escape. Turn the temperature right down as in your recipe and leave it to cook long and slow - the more time you give it the better ... depends if you can afford the electricity/gas :-). You can turn the lamb if you want then, but only once and always reseal it very tightly in the roasting dish. It should be so tender after that, it will fall apart. You can also wrap it in the foil to rest for after cooking. I hope that helps.