I've tried many different formulas for cooking leg of lamb and Jamie Olive wins hands down IMO. His method is also very quick and has the advantage that you can do the potatoes at the same time as the oven is very hot.
You pre-heat the oven to 225 degrees C / 425F / gas mark 7 and cook it for:
10 minutes for every 450g/1lb plus twenty minutes for PINK
13 minutes for every 450g/1lb plus twenty minutes for MEDIUM
20 minutes for every 450g/1lb plus twenty minutes for WELL DONE
Turn every 30 minutes.
I like roast parsnips and minted carrots with lamb. Mmmmm...