Hi
Thank you for your advice. Appologies for not getting back but the response got filed into a side box by my email.
On reading your reply I had another go at troubleshooting the issue and noticed that other people had managed to sign up to the newsletter through the other sites even though I was unable to which seemed strange. I found that if I am not logged in as admin for the sites your plugin works fine. Since no one signing up to our newsletter would be logged in as admin for practical purposes there is no problem. Thank you for your time and plugin.
PS It also seems to be related to the WordPress MU Domain Mapping plugin.
bw