Sorry for the delay in getting back to you.
Thanks for your help, I have found that deleting web history in IE8 and Firefox has helped restored the ability to delete messages, however it is not perfect everytime.
It does also seem that I get different results according to whether I log into fb from my page where the application is uploaded or if I log in to fb directly to the site.
It is possible to post using this app without being logged into facebook, the posters appear as anonymous.
Perhaps the 'quirkyness' is something do with the fb end of things!