Yup - that makes a difference - stick a skewer through it a couple of times to make sure that it isn't frozen, if it is frozen - then seal it in a plastic bag and put it in a sink of luke warm water for a bit to get it to thaw quickly. It is really important to get it up to room temperature before you try to cook it....don't roast straight from the fridge.

In order to get it to medium you want to get meat up to only 70 degrees, any more and it will become well done. The faster you cook it, the more unevenly it will cook as the outside will cook first - slower is best - I usually roast beef joints at 80-90 degrees for 6-7 hours.

For your joint - rub some cracked pepper into it then sear it in a frying pan with some hot oil to seal in the juices, then pop it in the oven at 160 degrees for about 70 minutes, then take it out of the oven, cover it with foil and let it sit for at least 15 minutes before you carve. It will keep cooking for this 15 minutes on it's residual heat.

If you have a meat or probe thermometer then you can check the core temperature of the beef and remove some of the guesswork.