bless! 40 mins per kilo plus 20 mins (1 hour 40 mins), and leave it to "rest" for 10 mins before carving. (check it's cooked through by inserting a skewer/sharp knife into the thickest part, usually behind the leg, to check the juices are clear. anything pink, back in the oven!)
I'm not much good at this kg malarky! I would suggest you re-weigh it as lbs and ozs. Then 20 mins to the pound, plus 20 mins and let stand for 10 mins before carving.