Roasting Time Calculator
Using these times and temperatures should give you a pretty good result, but calculated roasting times can not take into account different shapes of joints, how cold the meat was when it went into the oven, or whether your oven temperature is actaully what it says on the dial.
For this reason a meat thermometer really is useful. Any cook shop stocks them, but if you get one with a wire so that it can be left in and read without opening the oven, it saves hassle. We got ours from Lakeland