Processionary Pine moth

We have been told we have these creatures in our garden on the pine trees. Does anyone know how to get rid of them as they seem to be munching their way through our trees, leaving sad brown branches. I have had conflicting advise from friends as to whether they are a notifiable pest, or just a nuisance we can deal with ourselves. Has anyone else got a solution? At the moment we have dozens of muslin like sacks hanging in the trees, generally on the higher branches.