Okay, i've got it working.
Basically 5GHz is
always on. I can't disable it. All I can do is either choose to give it a separate SSID to the 2.4GHz signal, or not.
The 2.4GHz signal only enables if I manually set the wireless channels on the AAPE. If I leave it on auto, it switches the 2.4GHz off totally.
And also, the iPhone cannot connect to any form of 5GHz signal on the AAPE. I've tried enabling it with only 5GHz and a/b/g only, but it still won't see it. That can be worked around, however, by using the 2.4GHz.
Thanks all for your help. I don't suppose any of you know why the AAPE works like this? For an Apple product, it's probably the most complicated router i've ever had to use. Even Linksys, with their pages and pages or sub-menus is more intuitive than the AAPE. Ah well.
[Edit] Shine, it's a brand new Apple Airport Extreme Base Station. Bought it yesterday, so I believe that makes it revision 5?