Thanks, I managed to answer my own question in the end, I had a version of iWorks on the machine and subsequently I got the latest version of Pages from teh App store. What seemed to be happening was that the document was being written in one version but associated with the other version. I removed both and re-installed from the App store and that seemed to resolve the issue. Why this only started happening recently when both versions have been on the machine for some time !
Anyhow, thanks for your response - maybe this will help others.