Jamie sure as eggs is egggs it'll be 20kgs mate...
You should have your tickets at least 2 weeks before you fly so do check them....Syill gives you plenty of time before you leave to get it right...
I read somewhere that it's about £15 per extra 5 kgs if you are over loaded,but to be honest if your going for just a week and are over your allowance then your taking to much anyways...