I wouldn't look at it like a Killing mistake, I would look at it more like a Learning progress, because from her on, you can ONLY get better. Just don't let this one hatching stop you from trying again. But we don't know whats going on yet so Lets keep happy thoughts...Thank you so much for your help and advice. I've dropped the humidity completely and will keep candling them until the air cell drops and let you know what they look like when I do. I'll make sure to let you know if there is any spaces that are yellow or not totally dark at the other end on that day.
They are Welsh Harlequins and I am feeling awful that I've made this mistake that will likely end up killing most if not all of them!
Really appreciate your help on this! And will let you know what I see on day 25.