There was an item on Radio 4 about this on Sat. They said that the frequency of trains to Gatwick would actually increase to one every ten minutes. Perhaps thats being economical with the truth as when you combine Gatwick express with South Central, there must be that many already.
So will the fare be £13 or £8 each way?
Will they try to sneak in stops at Clapham Junction?
At the present. personally, I'd save a fiver and take the South Central unless there was a long wait. Its no slower at peak times.
As for the Heathrow Express. "Famous for Fifteen Minutes". The implication being that it takes fifteen minutes, when in fact its more like 20. Thats going to get worse when the Terminal Five service starts. Are all trains going to Terminal five? If they are the service is going to need a shedload more trains, so price increase on the way.