That's a good way, but start with ...Sir, as you will already have his name and title in the correspondance address! keep it short and to the point, don't bullshit! no more than 3 paragraphs...Intro ....explain reason....finish!!
Better make the reason a good one, 'cos he will check as i take it from the fact that you feel you need to write a letter in the first place that it is the Mess's Christmas Ball that you cannot attend or a three line whip function!!