ok here is an update!
I tried booting just from the installer and tried selecting the option to reinstall OS X and nothing else. However,
that was also met with an error message...argh.
So what I did was take out the drive, put it in a USB dock and using ANOTHER Mac then ran Disk Warrior on it. This spent around 25 minutes doing it's stuff.
When repaired, the disk was put back in the original Mac and hey presto the the OS X install went all the way without any error messages.
Yosemite has now been installed. Lesson: In-built Disk Utilities are limited in what they can do and Disk Warrior is amazing.
Thank you V for your help here. It was much appreciated.