Heathrow to Gatwick

Take the tube. Catch the Picadilly line at Heathrow. Change at either Earl's Court, Glouster Road or South Kinsington to the District Line (headhing toward Westminster) and get off at Victoira. Then go upstairs to the main National Rail Train Station and take the Gatwick Express. According to TfL (Transport for London) it should take about 1 hour 40 min. to 2 hours. Here is the website (journeyplanner.tfl.gov.uk/) where you can check this and other alternative routes.

For most departure times the direct National Express bus ride is scheduled to take 1hr15min (sometimes 1hr35min). If you allow up to one hour to clear immigration/passport control at Heathrow, one hour plus for the bus ride, and one hour to get through security at Gatwick, then you get a bare minimum connection time of three hours. Unfortunately, unless your plane arrives on time at Heathrow, you get through immigration/passport control in a timely manner, and your bus doesn't run into traffic jams on the M25, you still are at risk of missing your plane at Gatwick, even with four hours to make the connection. The odds are in your favor; but if your plane is late, then all bets are off.

Jennifer I've never known the bus to take much over an hour. I can't guarantee it of course. I guess with an accident on the motorway you could be unlucky, but barring that the bus is certainly quicker than going via Victoria. A hire car or taxi probably does the journey, again in normal traffic conditions, in around 40 minutes so you might want to consider that if your timings are really tight. Alan

I did the reverse the Saturday before last and after a bit of research decided booking a car was the only realistic option. It worked well and cost 55GBP. The journey was supposed to take about 45 mins but took closer to 1hour and that was late Sat afternoon with light traffic. I am sure it is perfectly doable otherwise but unless I had a lot of time and little luggage I would do the car every time.

And it takes time to get to the bus station from immigration. It may be a while before the next bus. Allow up to an hour (or more) for this. Honestly, 4 hours may not be enough time if you use public transportation.

Here is an alternative public transport route, which is little different in time with about a 10 minute frequency: Bus 285 from Heathrow Central to Feltham station South West Train from Feltham to Clapham Junction
Southern Train from Clapham Junction to Gatwick. You are still looking practically at close to 2 hours with any indirect PT route though. If it isn't a separate ticket though you should be able to be put on a later flight (space permitting).

Thanks for all the help so far. I got the three hour bus timing from Heathrow's website. That's included with waiting for the bus and traffic that may be encountered. Liz, which car hire or taxi did you use for 55 pounds? I saw one off Gatwick's website that was 81 pounds (wow) so I'd take 55 over that for sure. Thanks again everyone!

I haven't used it myself, but one highly recommended private car service is Just Airports. They give a discount for paying cash. Their site doesn't list the fare for Heathrow-Gatwick, but you can send an e-mail and ask for a price quote.

Tim, thanks I just sent them an email!

This is the one I used http://www.airport-pickups-london.com/ one thing to remember if you are booking a car - add at least 45mins to an hour to your arrival time. It will likely take this long to clear passport control and collect your luggage- possibly longer, even if the flight is on time.

Thanks Liz that does seem to be the cheapest I can find at this point :)

Things have not been running well the last few weeks at Heathrow with passport control either so the more time you can allow the better.

If you do decide to take the Tube, the easiest place to change from Piccadilly Line to District Line is at Hammermith. All you do is walk across the platform as this is one of the few places where both lines are on the same level. After hammersmith the piccadilly line goes deep underground, so changing at one of the other locations will require stairs or escalators. (You will find this advice recurring on this site).