I've cracked the centering. I needed to tell the software I specifically wanted to use it with Word documents.
I'm finding it very useful and the accuracy is very good. The manual isn't very easy to find things in, so I haven't got the hang of some of the formatting I need yet. I haven't done a long session of uninterrupted dictation yet as I'm using it for a university assignment and researching the information as I go, so I can't give you any useful timings at the moment (I could do something for you next week.). I've also found out how to open and save documents by voice alone, which is a nice extra.
My husband who is multiply disabled has now bought himself a copy. He's completely deaf, but his speech isn't too bad, so I'll be interested to see how easy he finds it. He's also a complete non-techie, so if he can make it work then anyone can!