The missing lines are
Ready to hear thy call
Ready to give thee all
Ready whate're befall
Mother of mine

This was also our school hymn. I went to an all boys school in Pimlico London in the 1950s   As you might imagine hymn singing was not at the top of the list for teenage boys but when we sang this they would have heard it streets away.   I now have alzheimers but can still recall this song and our singing it and cherish that.