Upon further investigation, I discovered that not only the user input had changed, but so had the time zone to Pacific Standard Time.
How did it happen? Not 100% sure at the moment, however we applied three hotfixes to WDM (Wyse Device Manager) the previous day, so have good reason to believe these updates were the cause.
As our WES7 thin clients were on the domain, the fix was pretty straight forward. This fix also applies to Windows 7 and Server 2008 R2 and may work on other, newer Windows OS's (untested)
To change the time zone, create a logon script to run the below as a BAT file:
tzutil /s "GMT Standard Time"
To set the user input (keyboard) to English UK, create a startup script, or use group policy preferences to push out the following registry changes:
Of course, for those of you reading this from elsewhere in the world, these can be changed for other input languages or time zones.