I prefer to cook poultry at 160C for longer than at a higher heat for a shorter period. I roughly give it 20mins per 500g.
So, I'd probably cook your turkey for 2.5hrs at 160C BUT ensure I gave it at least an hour to rest in a warm spot. I'd also baste it every 30 mins during cooking. My favourite turkey basting mix is equal parts butter to maple syrup.
I just read the other replies and wanted to explain the reason for the big difference in my cooking time suggestion. I read the OP as having a 'buffe' roast so it would be a rolled piece of breast with no bone. A roast with bones would take longer to cook through due to the density of the bone vs meat. The meat will keep cooking as it is 'resting' so it can come out of the oven slightly underdone. The internal temperature will still rise a few degrees if kept in a warm spot to gently relax.
If you use a meat probe, leave it in until serving time because otherwise you'll allow all the precious juices to escape. Turkey tends on the dry side anyway so you want to minimize that moisture loss. Same reason why I don't recommend skewering meat to look for clear juices as a sign of done-ness.
Take a leap of faith with timing (if you can trust your oven temp) and if worse comes to worse and it is still raw when carving, chuck it in the microwave to zap it or pour over gravy and heat it up in the oven.
And edit again to explain that basting time should not be included in cooking time. If you baste it 6 times during that period and it takes 5mins each time, the overall cooking time will be plus an extra 30mins.
This post has been edited by *LucyE*: 21/12/2012, 11:10 AM