Posted: 09/08/2014 at 19:29
Not a Phormium but a Yucca ( if the leaves have a sharpish tip to them...not all yuccas have stiff leaves ) or a Cordyline.
Your suggestion of gradually raking the soil away from the base will hopefully give you a clue . A reasonable estimate for the root spread would be to start from the point at which the leaves spread out to....the canopy often reflects the root system.
If you want to transplant rather than dump the plant, you could remove a lot of the lower leaves which would make it easier to deal with and give it a better chance in it's new site. It does look rather large but as big a root ball as possible and have the new site ready prepared.
Hope you are successful.......looks a nice healthy plant so you should have a good chance of being successful.