Thank you for all of the hints and advice. I have modified /etc/sysconfig/kernel and again altered the CentOS-Base.repo to fetch the kernel from the mainline repository rather than from Plus. I cleared out the rmp database kernel entry and installed the "real" kernel via yum, checked that grub.conf was ok, rebooted and all seems good.
Can someone point me towards a document detailing the function, settings, and syntax of /etc/sysconfig/kernel. I tried to locate this via google and apropos but came up with just a few oblique references and no substantive documentation.
Regards,