Thanks for the replies - I strongly suspected that root surgery was not an option! but raising the patio is going to be a BIG job - unfortunately, replacing the affected flags with benches / planters etc. won't do the job - about 20% of the flags have lifted to some extent along two differnt lines. Might be able to remove a couple of flags closest to the tree. Likewise, moving the patio is not an option - it is trapped between the tree and a wall on the south, the house on the northe, a wall on the west and the conservatory on the east!!
If I do raise the whole patio it will involve a 3-inch layer of sand accross the whole area - I'm a bit disturbed by the suggestion that this might in itself damage the tree :( I'm pretty certain the tree now has it's full growth and no further movement of the patio surface has taken place for a couple of years.