Hi Lucy and welcome to GoY. I think you will find digging up a tree of that size requires a digger. The root system will be at least as large as the canopy, possibly larger. If you do to much damage to the roots the tree will die. If you must move wait until autumn and be prepared to water very, very well for the first year.

This is going to be one heck of a job!! The root system of a tree is usually at least as big as the top growth that you can see.
There are instructions here -
http://en.wikipedia.org/wiki/Digging_trees_and_shrubs_for_transplanting

I heard on Gardeners Question Time several years ago that ideally you should start to plan a move at least 2 years before you do it. The first year you cut halfway round the tree, in a semicircle, as deep as you can go and where you'll want the soil and root ball to end when you lift it. Keep the tree well watered if it's dry - you want it to form a smaller, denser root ball on this side. The next year you do the same thing on the other side and allow a year for it to make new roots on that side. THEN you try the lifting thing, but I think, with a tree that size you'll be needing lifting gear / a crane plus a digger.