Cook for 20 minutes per 1lb + 20 minutes; e.g. if the joint weighs 3lb that is 3 × 20 minutes + 20 minutes = 1 hour and 20 minutes
Meanwhile either go to the pub for a while, have a few beers and chips on the way home, on your return, remove what is left of the lamb from the oven, hack off a few chunks and place between thick slices of white bread then go to sleep in front of the television
Alternatively, peel a few spuds, slice some carrots and cut up some greens
30 minutes before the meat is ready boil some water to cook the potatoes (20 minutes)