Direct Bus Service??
Wangi: In the language of National Express, "Direct" means VIA HEATHROW !!! Although the schedule states 3hours, don't count on it if you have a flight to catch.
If feasible (if you live to the west of Stansted) - I would drive to Luton Airport Parkway station, and get on one of the half hourly direct Thameslink trains to Gatwick - they take about 1h30, and get pretty packed in Rushhour but I think it is almost the only way to get from North to South in anything approaching an acceptable time (apart from going by car the middle of the night - when you are less likely to run into traffic!!)