Originally Posted by ajamieson

That's a very unrealistic estimate. Even if you know all the cheats and rat-runs then 20 minutes is unlikely. I've done it in 15 as a fluke but I wouldn't rely on repeating it.

Assuming that you don't get into the complex game of treating yourself as an arriving passenger and departing from scratch at T1, then you've got to consider:

• The arrival gate at T3, which chould be a 10 mins walk from the main building
• Waiting for the bus to T1
• Security at the Flight Connections Centre, which can take anything from 2 minutes to 45 minutes
• Immigration at the Flight Connections Centre, usually okay
• The long walk to along to T1 departures
• Sorting out updated boarding passes etc