I thought I could just delete the original attachment, which edit topic allows, and replace it with the new one however it appears that adding an attachment is only available at the point of creating a new message not when editing ...
I want that too.
?
... so I've resorted to creating another new topic is this by design??
No, I think Mark just forgot (see his reply in that topic). Maybe if you add a "bump" to the topic he'll find the corresponding round tuit.