Start by removing the leg from the fridge at least an hour before cooking to allow it to come to room temperature. Rub the leg with oil and season it well with salt & pepper. Preheat your oven to a high heat, 220˚C/200˚C fan/gas 8. Roast the lamb for 25 mins until golden brown all over. Reduce the heat to 190˚C/170˚C fan/gas 6 and continue to cook for another 55-60 mins. This should give you a medium/rare blush to the meat. Remove from the oven, cover in foil and leave to rest for 20 mins before carving. If you have a meat thermometer then 52-54˚C is rare, 60-65˚C for medium and anything above for well done.
To get a bit more flavour into the meat you can cut 15-20 deep slits into the flesh and stud them with some sliced garlic cloves and rosemary leaves. A few crushed anchovy fillets work wonders with lamb too.
For a good gravy roast the leg on a trivet of onions, carrot, celery and stock. After resting the meat, strain the liquid and skim away the worst of the fat. It can be enlivened with a dash Worcestershire sauce or a blob of sweet red currant jelly or Cumberland sauce.