Richard.
Your invalid ID notice is caused by you making posts with the same title "Venue Change" for example. Deleting the post does not remove the URL. It's the conflict in URL causing the problem.
The Venue Change subject was merged with the topic of G.S. Hall. Please don't post "orphan" subjects, when they can be easily included into existing topics.
OR
Use the Announcement Feature in the group section if it is a one-off notification - please do not post to forum.
Same applies to making "test" posts using the same title - please don't
To Casper Mist - I haven't deleted any of your posts - ask the moderator of the Group whether they have - but if they have:
To group forum moderators - please do not delete posts in your group posted by other people unless it violates the forum Terms & Conditions (offensive, abusive etc - you get the picture).