Posted: 07/05/2013 at 14:23
Thank you everyone. Ijust called the RSPB and they have assured us that we have done the right thing, to prevent them from nesting there. Once they were nesting we would be unable to touch them until they had migrated back to Africa.
They said that once the swallows realise they can't get to the old nests they will make use of, and repair the numerous redundant other swallow nests around the site. They have also suggested that we make lots of mud available to the birds to facilitate the nest building. I did this this morning and have already spotted the swallows making use of it.
Thanks for all your comments folks. I though I would let you know the outcome just in case anyone else has the same problem.