I found an egg - hers - in the Run this morning. I found her in the nestbox, sitting on one of Delilah's. I retrieved the egg, gave Scarlett a cuddle, and put her outside. She ran round, up the ladder, and into the nestbox again. I hoiked her out, put her on the ground, and shut the nest box door.
She's laid every day for the last 10 days (and has laid 13 eggs in the last 15), so she thinks she has a clutch that needs incubating.
Although we've spotted it early, I'm not sure gentle pressure (like blocking access to the nestbox) will work. She's already tried to fly on top of the Cube today. Looks like we'll have to di out the broody cage.