navigation question

Was toying with my gps recently and plotted a course from my home in the
midwest to London Heathrow. I know it uses great circle routing to
calculate the distance and course. Going from west to east it shows a
course of about 45 degrees. On the return trip it shows an intial
course of 310 degrees.

Now, to my old mind, I would have thought a reciprical course of 225 .
It is something in the spherical formula used for the great circle but
I'm too rusty to work through it.

An old navigators who want to help me out?

Thanks in advance.

MAH