We watched the box again yesterday evening - and saw one juvenile again. He just sat on the ledge at the door for about 45 mins, then all of a sudden got quite agitated and started calling. It was as if he was responding to some sort of signal.
He started leaping about the shelf at the front of the box and progressed to jumping out to a branch in front of the box, then back to the shelf, then to a trunk to the side of the box and from there onto the roof of the box. From there he went out to another branch, and then attempted to get to another branch further away but didn't make the landing so took a tumble to the ground - but seemed to be ok cos he then got on top of the wall by the gate. We saw an adult come in to the area where he had ended up. Later in the evening (our evening walk with the dogs around our field) we could hear the juvenile making its distinctive 'feed me' calls from some trees on the other side of the lane to us so we are surmising that he was being persuaded by his parents to go further down into the more wooded area at the bottom of our valley/dell - most probably their hunting ground.
We will keep an eye out for them tonight but don't think they are in the box anymore.
The photos show the juvenile perched in the door of the box - they were taken in low light and i have brightened them in photoshop.
BTW - the nest box we made is to the plans for a barn owl box!