iMac 2008 boot issue

I 'inherited' a 2008 iMac from my wife recently after she upgraded her desktop machine.
I hated OSX, so I've wiped the drive and installed Xubuntu 13.10.
This is the hardware info:

24" iMac, 2.8GHz Core 2 Duo, 4GB SDRAM, 320GB Drive.
ATI Radeon HD 2600 PRO with 256MB of GDDR3 memory

The drive is partitioned as follows:
/dev/sda1 - fat32 - boot
/dev/sda4 - ? - bios_grub
/dev/sda2 - ext4 - /
/dev/sda3 - swap - swap
/dev/sda5 - ext4 - /home

I'm pretty sure I have the boot screwed up, but it does boot however:

on turn on, it boots into GRUB2 in command line mode.
(I get the grub> prompt)

if I type exit, it closes and boots into rEFInd (I installed this in the mistaken believe that it was required)
Here I can select one of three options - two take me back to the GRUB command line and the third boots me into Xubuntu.
This machine is not dual booted, there is just Xubuntu and nothing else.

What have I done wrong and what should I do to get this working correctly?