gatwickexpress.com/virgin is better - 5% off and a free upgrade to First Class, although you need to book by 31 May

Feel free to change the Virgin bit in the URL above to other airlines and see what turns up!

You know that its far cheaper, though, to take the local First Capital Connect train? This stops in Kings Cross, Blackfriars, London Bridge and other places and only takes 10 minutes longer. Unless your hotel is near Victoria your overall journey time is probably quicker on this train.