Thanks Hawkwalk. I would have thought the evidence from the energy savings trust etc would be enough. However, our neighbour reads the Telegraph and has shown us cuttings from Jeff Howell's On the Level column, who is staunchly against cavity wall insulation: (
http://www.askjeff.co.uk/content.php?id=4). I think he will continue to believe the newspaper columnist's opinion rather than be swayed by any other evidence.
I guess it's not worth falling out with our neighbour over, but my husband and I are very "green" minded and feel this is one of the most effective things we can do for the environment and it's a shame we are unable to proceed unless we have our neighbour's agreement.