I think I'm seeing courtship behaviours!
The two birds flew back to the nestbox. The smaller (male) entered first, then the female followed, facing him. He bowed lowed, bobbing his head, down into the gravel, keeping it very low all the time (in what might look like a submissive position if they were dogs, not birds). He used short squeaks and peeps.
She faced him in the nestbox, keeping her head high, but sometimes bringing it down low to his, beak to beak. She screeched up a storm. Lasted about 5 minutes.
Is this part of the "scraping" courtship behaviour mentioned?