Roasting times are all very well for your joint of beef or lamb, but there are so many variables - whether you have a fan oven, whether your oven heats up fast once you've popped the joint in, maybe the door seal is slightly worn so it doesn't keep it's heat as well as it should, maybe the wind is blowing down the flue of your aga trying to blow it out - that definitely doesn't help, and then there's the hole in the glass of your cooks perks.....
Yes, so many variables, but the one thing that's utmost in your mind is that you don't want to either 1 overcook your meat ("get the serated knives out") or 2 undercook it (especially in regards of chicken) and leave elderly or young members of the party (who are more susceptible) at risk of food poisoning.
The answer is simply to use a food temperature probe as I do for all my cooking.
You can buy these at most kitchen shops. You just have to stick the probe into the middle of a joint or cut of meat, wait for a couple of seconds for the temperature to adjust. Don't probe next to a bone - they act as radiators for heat, so the temperature reading will be inaccurate.
For beef and lamb I work on:
52 oC rare
58 oC medium
68 oC+ well done
For Chicken it should be 74 oC
For pork 74 oC for 15 seconds. This may sound high, but cooking to this temperature ensures it is safe (there is a small possibility of undercooked pork leading to Trichinosis though such cases are now thankfully rare).
There is more information on cooking temperatures here: http://www.cooksrecipes.com/tips/meat-cooking-temperature-chart.html