I got a reply from the breeder, and she said that if the weather is hot then the hatching will happen earlier. She has known pipping to take place earlier than day 25 in her eggs.
The crack is like a stone chip on a car windshield, i.e. a central point with lines radiating outwards. 2 of the lines are long and go around the end of the egg, but do not meet. There is no hole yet, i.e. not broken through shell.
Broody hen sits religiously all day and night so the eggs are kept pretty warm. I have to take her off every day to make sure she gets food and water. We have had some cool days but also some very hot ones (for England that is!) Today is going to be around 85 degrees.
Checked this morning, and the same as yesterday. I did try candling a couple of days ago (before I saw cracks) but could only see dark blob and air sac at end. Could not make anything else out. No cheeping, squeaking, tapping or other noise coming from eggs.