One of the most useful features in the ever expanding tool set that is Google Now is the 'OK Google' Hotwords. Unfortunately many people are discovering that this handy feature is not active outside of the US. With Google releasing an update to their iOS app over the last couple of days this annoyance has spread to Googles iOS users too. However there does appear to be a handy fix.
When you go into the application settings you'll notice that it's impossible to activate Hotwords. Changing your language in the application won't make a difference. However, go to
iOS Settings -> General -> International and update your language to plain 'English'. Once the phone has finished applying it head to the Google app and Hotwords should now be on by default.
I haven't found any issues with this so far but please be aware that I accept no responsibility for any damage or other inconvenience caused to your device or yourself through following this tip. I'm offering it simply to let you know how I got this working on my iPhone 5.