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.