Okay, latest update:
Tonight was lockdown time for the next four eggs, so I used the opportunity to candle everybody in the bator. As well as the four eggs on day 18, I had another 10 on day 6. The four eggs going into lockdown are still looking pretty good overall, but two of them I'm feeling a bit iffy about. They just look a bit... not right. The membrane at the fat end of the eggs (just before the air sac part) looks weird. Almost like there are burst blood vessels in there. I've marked those two, so I'll be able to tell them apart later on...
The ten on day 6 gave me a pleasant surprise. I wasn't expecting too much from them cause the oldest were a full fortnight old, but 9 of them are fertile. Woohoo!
So I've removed the infertile one, removed the turning rails and put the four prospective hatchers fat end up in an egg carton. These four had to cope with extra high humidity between days 3 and 10 due to my first six eggs being in lockdown, so from days 10-18 I took the water trays out completely to make sure they lost enough moisture. Humidity was running at 30-35% most days, and occasionally made it as high as 40%. I'm aiming for 80-85% for the next few days as I'm going to be opening the bator to hand turn the others four or five times a day, and I want to keep the humidity as high as possible in there.
This is getting exciting now!!!
Edited to add: The infertile one I removed wasn't infertile after all. I cracked it into my dogs bowl and it was a goopy liquid yolk with strings of blood in it. Blood ring! Yuk!