OK, I went with Leigh's suggestion. rpm -e --nodeps grub-0.97-13 then yum install grub, and I still get the same error on the grub command.
grub: asmstub.c:214: grub_stage2: Assertion `simstack_alloc_base != ((void *) -1)' failed.
So I guess I'll go with the upgrade option from the install screen. It's worked before so it should work again.
I'll try some reboots first to see if I really need to. (See what boots or doesn't boot).