I switched from UK to Global firmware. My N900 now no longer works with channel 13 (perhaps 12 too, not sure). In fact, I'd already switched to Global FW, then moved my wifi from 6 to 13. The N900 followed the wifi up to channel 13, but when I rebooted it, it wouldn't find the access point. I'm now using channel 11. Hope this clears things up a bit, although I don't understand why the N900 with Global FW followed the access point and worked when it changed from 6 to 13 initially, before the reboot. Perhaps that is a bug.