Thanks Reezekeys
I have around 20gb of free space and have been deleting cache files, duplicates, download folder etc.
When I said I have a TM backup I wasnt suggesting I would re-install from this, just that I have it as a "backup" if everything goes wrong or I need to retrieve something that I forgot I needed.
That is interesting about you changing the drive. I didnt think the drives could be changed on these models?
Thanks for the tip on bootable USB stick