Keyboard keeps reverting to U.S

For a few weeks, I've noticed that my keyboard layout keeps reverting to U.S. each time I reboot. To be precise: each time I reboot, my System Preferences -> Keyboard -> Input Sources window looks like this:

I keep changing it to this (but it doesn't stick around for long):

NB: MathUnicode is a .keylayout file of my own making, which I've used for several years now, without any problems.

I'm running OS X Yosemite (10.10.1) on a MacBook Pro (13-inch, Early 2011).

Does anybody know how I can stop it reverting to the U.S. keyboard layout?