[MickDoneDee]

I decided to do it directly within the database.

First I created the post within the forum by using New Thread and copying the contents of the post to be copied into the new threadstarter.

Then I went directly into the database and replaced my details with the details of the original poster.

In the post record I replaced these fields: username, userid and ipaddress.
In the thread record I replaced these fields: postusername, postuserid and lastposter.

All the other fields were filled correctly because I had created a new thread and post.

Finally: in ACP -> Import & Maintenance -> Update Counters -> Rebuild Forum Information

Still not a quick operation but saves copying a long thread just to split one post out of it.