Just for completeness and providing as much info as I can, there is a small 'hickup' in the channel 13 solution.
What Sony has implemented, means that the WiFi startup process checks the mobile carriers 'mobile country code' which is provided in the mobile signal header.
If the Mobile Country Code is registered in the WiFi startup, it allows channel 13 to be activated.
Once you have your Mobile Country Code registered in the WiFi startup you're good to go and you'll see WiFi channel 13.
The only problem to all this is if you disable WiFi, enable airplane mode and then enable WiFi again.
The phone is no longer receiving a Telephony signal and thus not seeing the Mobile Country Code, so WiFi Channel 13 access is dropped.
If you disable airplane mode, channel 13 is accessible again, so there is some small amount of dynamic checking in a few scenarios.
A better solution is to switch the code such that it'll give you WiFi channel 13, unless it's an North America Mobile Country Code.