Andrew and Eric, many thanks for the replies. I have run the diags and it is fine. Most differences are below 1. What got me going was a set that had a few above 1. It was then that I started wondering why I would get such a result.
Funnily enough, the last scan that I did was with the three pages from the 1728 Atkinson target sitting on the table. It was the cleanest scan that I have got.
I would love to get some guidance on whether folks average, and, if so, how many passes is a good starting point.
Thanks for humouring a newbie. This is quite a learning curve.