Depending on size of cut -
Once oiled, peppered and salted -
roast in the oven for around 2 hours (in a pan and in tin foil)
after around 1hr.30 take foil off and cook for 25 mins more in open oven
then for the last 5 - 7 mins of cooking turn off the over and On the grill to crisp the top layer of lamb, leaving the inside tender and juicy.