From this point we started a little battle. I would take her out of the nest box and encourage her to join the others in exploring the garden. She would wander round outside for a bit and then disappear back inside again. On Thursday afternoon I shut her out of the nest box for a few hours. She didn't seem phased by this at all, but returned to her dark corner within minutes of me opening the pop hole.
So it came to drastic action. We decided to try and break her of this bout of broodiness. A hens determination to sit on some eggs can often lead to illness from refusing to leave the nest to eat or drink and we didn't want it to go this far.
As far as I can tell there are a few methods of doing this and all centre on cooling the hen down. Her hormones will raise her temperature to help keep the eggs toasty, so by cooling her down you trigger the end of this process - as if she'd left the nest of her own accord.
A popular method is to put them in a separate cage (like the sort of thing used during puppy training) and raise it off the ground so that there's a cool air flow all around. You'll probably need to do this for a week or so. We don't have such a cage, so skipped that one and moved onto an easier option.
We planted ice cubes in the next box. I'd love to have seen her reaction when she first discovered them. As far as we can tell she probably sat on them for most of the day, she was certainly still there that evening. The nest box was a soggy mess and we were all ready for a repeat the next day.