OK, after a bit of sleuthing I think I'm making progress.
I tried the 12.04 and 14.04 installers and I have the same problem as I mentioned at the start - disk partitioning fails.
I noticed this time that I have /dev/sda and /dev/sdb as expected, but I also see /dev/md127 in the list. This is the software RAID partition set up by mdadm when the original 14.04 LTS system was installed. The installer isn't removing it and since everything is configured as a RAID system, it is active and doesn't want its partitions messed with. Apparently the installer doesn't override this.
What I did was switch to the terminal, deactivate the RAID system, zero out the superblocks on each drive, and then modify /etc/mdadm/mdadm.conf such that it does not list any ARRAY configurations. After that I can now get through the installer for 12.04 without it complaining - it is installing now.
BUT, I see now option to set up a software RAID 1 system through the installer for 12.04. I haven't looked at 14.04 but from what I'm finding I don't expect it to be there. The next question is, can I set up a RAID 1 system after the install has been done on /dev/sda? My system as 2x2TB drives that I had planned to use in a RAID 1, and I'd like to keep with that, but can I actually set it up after the fact?
Once I get through this and see if I can get 12.04 working, I'll give 14.04 a go and see if it works on my setup as well.