Originally Posted by britz

It sounds like you want to head to the T3 lounge before heading to T5 - if that's correct just follow signs to T3 connections. You'll re-clear security (and inevitably be reminded your connection is in T5- just tell them you are going to the T3 lounge before going to T5). When you are ready to go to T5 there are signs for connections to T5. You'll more or less go to the same place you were before re-clearing T3 security. You won't have to go through immigration anywhere during the process but you will have to re-clear security entering T3 and again at T5.

It's a hassle but galleries F is awful for food and if you're wanting a bit to eat, the CX F lounge is pretty good comparatively.