You will need to get a much heavier piece of meat than if you were buying beef or pork if you are buying a leg of lamb because of the bone.

Ingredients

2-2.5kg leg of lamb
1 tbsp oil
Salt and pepper 

Equipment 
Ideally a meat thermometer

Method
Unless you buy your meat pre-packaged from a supermarket which gives you an exact cooking time then you will need to calculate how long the cooking process is going to take.  The times for cooking a joint is 15 minutes per 500g for medium and 20 for well done.  Using this calculation this means that for your basic 2kg leg cooked to medium would start at a fan oven temp of 200c for 20 minutes and then for 1 hour at 180c.  A 2½ kg joint should take 1 hour 15 minutes. 

Heat the oven to 200c fan. Rub the oil into the lamb and then season with salt and pepper.  Put the lamb into the roasting pan and after 20 minutes turn down the oven. 

The most accurate way to get your lamb to the way you like it cooked is to use a meat thermometer where you place the probe into the middle part of the joint (but not touching the bone) near to the end of the cooking time, this allows you to see how it is cooked exactly.  Your lamb will read 60c for medium, and 71c for well done. You can pick a basic meat thermometer, like the Kitchen Craft one, up on line for £5. 

Remove the lamb from the pan and loosely cover with foil, for 20 minutes.