Gatwick to Heathrow tranfer 9:00 Saturday morning

I've done it from deplaning in Gatwick to check-in at Heathrow in two hours, but that was with a UK passport and no traffic problems. You may be OK, but I certainly wouldn't count on it, and the American Airlines gate agent that day told me I needed to allow four hours. 90 minutes is out of the question.

It won't take 90 minutes. It will be much more. There have been discussions here previously on this, the archive search can help you locate them. Short story long, assuming you are arriving trans Atlanic (I've never heard of alington - I know of 3 Arlingtons) your first hour or so will be occupied in getting off the plane, baggage reclaim, immigration and finding the National Express coach stop. The coaches run every 20 minutes or so (have a look at their website ) and then take 1:05, 1:10, or 1:20 or so depending on which terminals at each airport, then finding your departure area, then you need +- 2 hours check in time. So, 9:00 + 1:00 (or perhaps a lot more) + 0:20 + 1:10 + 2:00 means for happiness your next flight needs to be after 1:30 pm. Ooops. Maybe a taxi could do it a bit quicker, say an hour. So 9:00 + 1:00 + 0:15 finding one + 1:00 +2:00 = 1:15 pm. Oops. Houston, we have a problem.