Good news, DaveRo, and probably less well-known than it should be. Assuming the Message_ID survives moving to a different thread, it can be considered a "permalink", "more permanent" than a thread ID. (Hopefully, the IDs of deleted messages are not being reused.)

The syntax you gave only shows the single message, which is not always what we want. We can click on the "Thread Name" link to get the entire thread (possibly thousands of messages), where we have to search again, or add the #... to the URL manually. Is there a syntax that beams us from the Message_ID right into its current thread context?

Perhaps there is a Mudcat thread explaining all this? I would not feel insulted if you pointed us there ("RTFM").