There are two train services, the Heathrow Express and Heathrow Connect. The first is faster and more direct. The second takes 10 minutes to get to central London and has only stops at terminals 1.2 and 3.
Gatwick Express (terminals 1,2,3,4)
Hours of Service:
Victory at Heathrow: 03:30 first train last train 00:30
Heathrow to Victoria: 05:10 first train last train 23:40
Service Frequency: Every 15 minutes
Journey time: 30 minutes
Ticket Price: 14.90 round trip 26.80
Destination Rail: Victoria - Central London
Buying tickets: Online (page Gatwick Express) and at the ticket desk of the station.
Gatwick airport shuttle
http://gatwick.airport-shuttle.com
First Capital Connect (terminals 1,2,3)
Hours of Service:
London King's Cross to Heathrow: Watch website first local connect
Heathrow to Kings Cross: Watch website first local connect
Frequency Service: 4 trains per hour
Travel time: 30-45 minutes
Ticket Price: 10 round trip 20
Destination Rail: Kings Cross
Buying tickets: Online (page Heathrow Express) and at the ticket desk of the station.
Local South (terminals 1,2,3)
There are three services between Gatwick airport and London through local trains:
London Victoria Service
The local trains carry the same route that the Gatwick Express. It is operating 24 hours 7 days a week. The journey takes between 40 and 45 minutes. The fares are much cheaper than any other.
London airport transfers
http://www.london-airport-shuttle.com
London Kensington Service
There is an hourly service from Gatwick to stop in West Brompton (Earls Court) and Olympia in Kensington. If you lose the train can pick it up at Clapham Junction in any of the services from Victoria stop here.
London Waterloo Service (Eurostar Connection)
Taking any of the frequent services from Victoria stops in Clapham Junction. In Clapham Junction can take any train going towards Waterloo taking about 10 minutes (there is one train every two minutes).
Ticket Price: 9 round trip 18