I think we went over this a while back and its a fascinating subject.
Anywhere north of Cheltenham is Grim for me (Its Grim Up North)
Grimes Graves, bank, ditch etc litter the countryside far and wide.
I remember reading a book called The Key about place names which I think was the one that associated Grim with Gog and MaGog the Cornish giants, I'll have to dig it out for another look.